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/*
* Written by Doug Lea with assistance from members of JCP JSR-166 Expert Group and released to the
* public domain, as explained at http://creativecommons.org/publicdomain/zero/1.0/
*/
package alluxio.concurrent.jsr;
import java.lang.Thread.UncaughtExceptionHandler;
import java.security.AccessControlContext;
import java.security.AccessController;
import java.security.Permission;
import java.security.Permissions;
import java.security.PrivilegedAction;
import java.security.ProtectionDomain;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Objects;
import java.util.concurrent.AbstractExecutorService;
import java.util.concurrent.Callable;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.LockSupport;
import java.util.function.Predicate;
/**
* An {@link ExecutorService} for running {@link ForkJoinTask}s. A {@code ForkJoinPool} provides the
* entry point for submissions from non-{@code ForkJoinTask} clients, as well as management and
* monitoring operations.
*
*
* A {@code ForkJoinPool} differs from other kinds of {@link ExecutorService} mainly by virtue of
* employing work-stealing: all threads in the pool attempt to find and execute tasks
* submitted to the pool and/or created by other active tasks (eventually blocking waiting for work
* if none exist). This enables efficient processing when most tasks spawn other subtasks (as do
* most {@code ForkJoinTask}s), as well as when many small tasks are submitted to the pool from
* external clients. Especially when setting asyncMode to true in constructors, {@code
* ForkJoinPool}s may also be appropriate for use with event-style tasks that are never joined. All
* worker threads are initialized with {@link Thread#isDaemon} set {@code true}.
*
*
* A static {@link #commonPool()} is available and appropriate for most applications. The common
* pool is used by any ForkJoinTask that is not explicitly submitted to a specified pool. Using the
* common pool normally reduces resource usage (its threads are slowly reclaimed during periods of
* non-use, and reinstated upon subsequent use).
*
*
* For applications that require separate or custom pools, a {@code
* ForkJoinPool} may be constructed with a given target parallelism level; by default, equal to the
* number of available processors. The pool attempts to maintain enough active (or available)
* threads by dynamically adding, suspending, or resuming internal worker threads, even if some
* tasks are stalled waiting to join others. However, no such adjustments are guaranteed in the face
* of blocked I/O or other unmanaged synchronization. The nested {@link ManagedBlocker} interface
* enables extension of the kinds of synchronization accommodated. The default policies may be
* overridden using a constructor with parameters corresponding to those documented in class
* {@link ThreadPoolExecutor}.
*
*
* In addition to execution and lifecycle control methods, this class provides status check methods
* (for example {@link #getStealCount}) that are intended to aid in developing, tuning, and
* monitoring fork/join applications. Also, method {@link #toString} returns indications of pool
* state in a convenient form for informal monitoring.
*
*
* As is the case with other ExecutorServices, there are three main task execution methods
* summarized in the following table. These are designed to be used primarily by clients not already
* engaged in fork/join computations in the current pool. The main forms of these methods accept
* instances of {@code ForkJoinTask}, but overloaded forms also allow mixed execution of plain
* {@code
* Runnable}- or {@code Callable}- based activities as well. However, tasks that are already
* executing in a pool should normally instead use the within-computation forms listed in the table
* unless using async event-style tasks that are not usually joined, in which case there is little
* difference among choice of methods.
*
*
Summary of task execution methods
*
* Call from non-fork/join clients
* Call from within fork/join computations
*
*
* Arrange async execution
* {@link #execute(ForkJoinTask)}
* {@link ForkJoinTask#fork}
*
*
* Await and obtain result
* {@link #invoke(ForkJoinTask)}
* {@link ForkJoinTask#invoke}
*
*
* Arrange exec and obtain Future
* {@link #submit(ForkJoinTask)}
* {@link ForkJoinTask#fork} (ForkJoinTasks are Futures)
*
*
*
*
* The parameters used to construct the common pool may be controlled by setting the following
* {@linkplain System#getProperty system properties}:
*
* - {@code java.util.concurrent.ForkJoinPool.common.parallelism} - the parallelism level, a
* non-negative integer
*
- {@code java.util.concurrent.ForkJoinPool.common.threadFactory} - the class name of a
* {@link ForkJoinWorkerThreadFactory}. The {@linkplain ClassLoader#getSystemClassLoader() system
* class loader} is used to load this class.
*
- {@code java.util.concurrent.ForkJoinPool.common.exceptionHandler} - the class name of a
* {@link UncaughtExceptionHandler}. The {@linkplain ClassLoader#getSystemClassLoader() system class
* loader} is used to load this class.
*
- {@code java.util.concurrent.ForkJoinPool.common.maximumSpares} - the maximum number of
* allowed extra threads to maintain target parallelism (default 256).
*
* If no thread factory is supplied via a system property, then the common pool uses a factory that
* uses the system class loader as the {@linkplain Thread#getContextClassLoader() thread context
* class loader}. In addition, if a {@link SecurityManager} is present, then the common pool uses a
* factory supplying threads that have no {@link Permissions} enabled.
*
* Upon any error in establishing these settings, default parameters are used. It is possible to
* disable or limit the use of threads in the common pool by setting the parallelism property to
* zero, and/or using a factory that may return {@code null}. However doing so may cause unjoined
* tasks to never be executed.
*
*
* Implementation Note: This implementation restricts the maximum number of running threads
* to 32767. Attempts to create pools with greater than the maximum number result in
* {@code IllegalArgumentException}.
*
*
* This implementation rejects submitted tasks (that is, by throwing
* {@link RejectedExecutionException}) only when the pool is shut down or internal resources have
* been exhausted.
*
* @since 1.7
* @author Doug Lea
*/
public class ForkJoinPool extends AbstractExecutorService {
// CVS rev. 1.344
/*
* Implementation Overview
*
* This class and its nested classes provide the main functionality and control for a set of
* worker threads: Submissions from non-FJ threads enter into submission queues. Workers take
* these tasks and typically split them into subtasks that may be stolen by other workers.
* Preference rules give first priority to processing tasks from their own queues (LIFO or FIFO,
* depending on mode), then to randomized FIFO steals of tasks in other queues. This framework
* began as vehicle for supporting tree-structured parallelism using work-stealing. Over time, its
* scalability advantages led to extensions and changes to better support more diverse usage
* contexts. Because most internal methods and nested classes are interrelated, their main
* rationale and descriptions are presented here; individual methods and nested classes contain
* only brief comments about details.
*
* WorkQueues ==========
*
* Most operations occur within work-stealing queues (in nested class WorkQueue). These are
* special forms of Deques that support only three of the four possible end-operations -- push,
* pop, and poll (aka steal), under the further constraints that push and pop are called only from
* the owning thread (or, as extended here, under a lock), while poll may be called from other
* threads. (If you are unfamiliar with them, you probably want to read Herlihy and Shavit's book
* "The Art of Multiprocessor programming", chapter 16 describing these in more detail before
* proceeding.) The main work-stealing queue design is roughly similar to those in the papers
* "Dynamic Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
* (http://research.sun.com/scalable/pubs/index.html) and "Idempotent work stealing" by Michael,
* Saraswat, and Vechev, PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186). The main
* differences ultimately stem from GC requirements that we null out taken slots as soon as we
* can, to maintain as small a footprint as possible even in programs generating huge numbers of
* tasks. To accomplish this, we shift the CAS arbitrating pop vs poll (steal) from being on the
* indices ("base" and "top") to the slots themselves.
*
* Adding tasks then takes the form of a classic array push(task) in a circular buffer:
* q.array[q.top++ % length] = task;
*
* (The actual code needs to null-check and size-check the array, uses masking, not mod, for
* indexing a power-of-two-sized array, properly fences accesses, and possibly signals waiting
* workers to start scanning -- see below.) Both a successful pop and poll mainly entail a CAS of
* a slot from non-null to null.
*
* The pop operation (always performed by owner) is: if ((the task at top slot is not null) and
* (CAS slot to null)) decrement top and return task;
*
* And the poll operation (usually by a stealer) is if ((the task at base slot is not null) and
* (CAS slot to null)) increment base and return task;
*
* There are several variants of each of these. In particular, almost all uses of poll occur
* within scan operations that also interleave contention tracking (with associated code sprawl.)
*
* Memory ordering. See "Correct and Efficient Work-Stealing for Weak Memory Models" by Le, Pop,
* Cohen, and Nardelli, PPoPP 2013 (http://www.di.ens.fr/~zappa/readings/ppopp13.pdf) for an
* analysis of memory ordering requirements in work-stealing algorithms similar to (but different
* than) the one used here. Extracting tasks in array slots via (fully fenced) CAS provides
* primary synchronization. The base and top indices imprecisely guide where to extract from. We
* do not always require strict orderings of array and index updates, so sometimes let them be
* subject to compiler and processor reorderings. However, the volatile "base" index also serves
* as a basis for memory ordering: Slot accesses are preceded by a read of base, ensuring
* happens-before ordering with respect to stealers (so the slots themselves can be read via plain
* array reads.) The only other memory orderings relied on are maintained in the course of
* signalling and activation (see below). A check that base == top indicates (momentary)
* emptiness, but otherwise may err on the side of possibly making the queue appear nonempty when
* a push, pop, or poll have not fully committed, or making it appear empty when an update of top
* has not yet been visibly written. (Method isEmpty() checks the case of a partially completed
* removal of the last element.) Because of this, the poll operation, considered individually, is
* not wait-free. One thief cannot successfully continue until another in-progress one (or, if
* previously empty, a push) visibly completes. However, in the aggregate, we ensure at least
* probabilistic non-blockingness. If an attempted steal fails, a scanning thief chooses a
* different random victim target to try next. So, in order for one thief to progress, it suffices
* for any in-progress poll or new push on any empty queue to complete.
*
* This approach also enables support of a user mode in which local task processing is in FIFO,
* not LIFO order, simply by using poll rather than pop. This can be useful in message-passing
* frameworks in which tasks are never joined.
*
* WorkQueues are also used in a similar way for tasks submitted to the pool. We cannot mix these
* tasks in the same queues used by workers. Instead, we randomly associate submission queues with
* submitting threads, using a form of hashing. The ThreadLocalRandom probe value serves as a hash
* code for choosing existing queues, and may be randomly repositioned upon contention with other
* submitters. In essence, submitters act like workers except that they are restricted to
* executing local tasks that they submitted. Insertion of tasks in shared mode requires a lock
* but we use only a simple spinlock (using field phase), because submitters encountering a busy
* queue move to a different position to use or create other queues -- they block only when
* creating and registering new queues. Because it is used only as a spinlock, unlocking requires
* only a "releasing" store (using putOrderedInt).
*
* Management ==========
*
* The main throughput advantages of work-stealing stem from decentralized control -- workers
* mostly take tasks from themselves or each other, at rates that can exceed a billion per second.
* The pool itself creates, activates (enables scanning for and running tasks), deactivates,
* blocks, and terminates threads, all with minimal central information. There are only a few
* properties that we can globally track or maintain, so we pack them into a small number of
* variables, often maintaining atomicity without blocking or locking. Nearly all essentially
* atomic control state is held in a few volatile variables that are by far most often read (not
* written) as status and consistency checks. We pack as much information into them as we can.
*
* Field "ctl" contains 64 bits holding information needed to atomically decide to add, enqueue
* (on an event queue), and dequeue (and release)-activate workers. To enable this packing, we
* restrict maximum parallelism to (1<<15)-1 (which is far in excess of normal operating range) to
* allow ids, counts, and their negations (used for thresholding) to fit into 16bit subfields.
*
* Field "mode" holds configuration parameters as well as lifetime status, atomically and
* monotonically setting SHUTDOWN, STOP, and finally TERMINATED bits.
*
* Field "workQueues" holds references to WorkQueues. It is updated (only during worker creation
* and termination) under lock (using field workerNamePrefix as lock), but is otherwise
* concurrently readable, and accessed directly. We also ensure that uses of the array reference
* itself never become too stale in case of resizing. To simplify index-based operations, the
* array size is always a power of two, and all readers must tolerate null slots. Worker queues
* are at odd indices. Shared (submission) queues are at even indices, up to a maximum of 64
* slots, to limit growth even if array needs to expand to add more workers. Grouping them
* together in this way simplifies and speeds up task scanning.
*
* All worker thread creation is on-demand, triggered by task submissions, replacement of
* terminated workers, and/or compensation for blocked workers. However, all other support code is
* set up to work with other policies. To ensure that we do not hold on to worker references that
* would prevent GC, all accesses to workQueues are via indices into the workQueues array (which
* is one source of some of the messy code constructions here). In essence, the workQueues array
* serves as a weak reference mechanism. Thus for example the stack top subfield of ctl stores
* indices, not references.
*
* Queuing Idle Workers. Unlike HPC work-stealing frameworks, we cannot let workers spin
* indefinitely scanning for tasks when none can be found immediately, and we cannot start/resume
* workers unless there appear to be tasks available. On the other hand, we must quickly prod them
* into action when new tasks are submitted or generated. In many usages, ramp-up time is the main
* limiting factor in overall performance, which is compounded at program start-up by JIT
* compilation and allocation. So we streamline this as much as possible.
*
* The "ctl" field atomically maintains total worker and "released" worker counts, plus the head
* of the available worker queue (actually stack, represented by the lower 32bit subfield of ctl).
* Released workers are those known to be scanning for and/or running tasks. Unreleased
* ("available") workers are recorded in the ctl stack. These workers are made available for
* signalling by enqueuing in ctl (see method runWorker). The "queue" is a form of Treiber stack.
* This is ideal for activating threads in most-recently used order, and improves performance and
* locality, outweighing the disadvantages of being prone to contention and inability to release a
* worker unless it is topmost on stack. To avoid missed signal problems inherent in any
* wait/signal design, available workers rescan for (and if found run) tasks after enqueuing.
* Normally their release status will be updated while doing so, but the released worker ctl count
* may underestimate the number of active threads. (However, it is still possible to determine
* quiescence via a validation traversal -- see isQuiescent). After an unsuccessful rescan,
* available workers are blocked until signalled (see signalWork). The top stack state holds the
* value of the "phase" field of the worker: its index and status, plus a version counter that, in
* addition to the count subfields (also serving as version stamps) provide protection against
* Treiber stack ABA effects.
*
* Creating workers. To create a worker, we pre-increment counts (serving as a reservation), and
* attempt to construct a ForkJoinWorkerThread via its factory. Upon construction, the new thread
* invokes registerWorker, where it constructs a WorkQueue and is assigned an index in the
* workQueues array (expanding the array if necessary). The thread is then started. Upon any
* exception across these steps, or null return from factory, deregisterWorker adjusts counts and
* records accordingly. If a null return, the pool continues running with fewer than the target
* number workers. If exceptional, the exception is propagated, generally to some external caller.
* Worker index assignment avoids the bias in scanning that would occur if entries were
* sequentially packed starting at the front of the workQueues array. We treat the array as a
* simple power-of-two hash table, expanding as needed. The seedIndex increment ensures no
* collisions until a resize is needed or a worker is deregistered and replaced, and thereafter
* keeps probability of collision low. We cannot use ThreadLocalRandom.getProbe() for similar
* purposes here because the thread has not started yet, but do so for creating submission queues
* for existing external threads (see externalPush).
*
* WorkQueue field "phase" is used by both workers and the pool to manage and track whether a
* worker is UNSIGNALLED (possibly blocked waiting for a signal). When a worker is enqueued its
* phase field is set. Note that phase field updates lag queue CAS releases so usage requires care
* -- seeing a negative phase does not guarantee that the worker is available. When queued, the
* lower 16 bits of scanState must hold its pool index. So we place the index there upon
* initialization (see registerWorker) and otherwise keep it there or restore it when necessary.
*
* The ctl field also serves as the basis for memory synchronization surrounding activation. This
* uses a more efficient version of a Dekker-like rule that task producers and consumers sync with
* each other by both writing/CASing ctl (even if to its current value). This would be extremely
* costly. So we relax it in several ways: (1) Producers only signal when their queue is empty.
* Other workers propagate this signal (in method scan) when they find tasks; to further reduce
* flailing, each worker signals only one other per activation. (2) Workers only enqueue after
* scanning (see below) and not finding any tasks. (3) Rather than CASing ctl to its current value
* in the common case where no action is required, we reduce write contention by equivalently
* prefacing signalWork when called by an external task producer using a memory access with
* full-volatile semantics or a "fullFence".
*
* Almost always, too many signals are issued. A task producer cannot in general tell if some
* existing worker is in the midst of finishing one task (or already scanning) and ready to take
* another without being signalled. So the producer might instead activate a different worker that
* does not find any work, and then inactivates. This scarcely matters in steady-state
* computations involving all workers, but can create contention and bookkeeping bottlenecks
* during ramp-up, ramp-down, and small computations involving only a few workers.
*
* Scanning. Method runWorker performs top-level scanning for tasks. Each scan traverses and tries
* to poll from each queue starting at a random index and circularly stepping. Scans are not
* performed in ideal random permutation order, to reduce cacheline contention. The pseudorandom
* generator need not have high-quality statistical properties in the long term, but just within
* computations; We use Marsaglia XorShifts (often via ThreadLocalRandom.nextSecondarySeed), which
* are cheap and suffice. Scanning also employs contention reduction: When scanning workers fail
* to extract an apparently existing task, they soon restart at a different pseudorandom index.
* This improves throughput when many threads are trying to take tasks from few queues, which can
* be common in some usages. Scans do not otherwise explicitly take into account core affinities,
* loads, cache localities, etc, However, they do exploit temporal locality (which usually
* approximates these) by preferring to re-poll (at most #workers times) from the same queue after
* a successful poll before trying others.
*
* Trimming workers. To release resources after periods of lack of use, a worker starting to wait
* when the pool is quiescent will time out and terminate (see method scan) if the pool has
* remained quiescent for period given by field keepAlive.
*
* Shutdown and Termination. A call to shutdownNow invokes tryTerminate to atomically set a
* runState bit. The calling thread, as well as every other worker thereafter terminating, helps
* terminate others by cancelling their unprocessed tasks, and waking them up, doing so repeatedly
* until stable. Calls to non-abrupt shutdown() preface this by checking whether termination
* should commence by sweeping through queues (until stable) to ensure lack of in-flight
* submissions and workers about to process them before triggering the "STOP" phase of
* termination.
*
* Joining Tasks =============
*
* Any of several actions may be taken when one worker is waiting to join a task stolen (or always
* held) by another. Because we are multiplexing many tasks on to a pool of workers, we can't
* always just let them block (as in Thread.join). We also cannot just reassign the joiner's
* run-time stack with another and replace it later, which would be a form of "continuation", that
* even if possible is not necessarily a good idea since we may need both an unblocked task and
* its continuation to progress. Instead we combine two tactics:
*
* Helping: Arranging for the joiner to execute some task that it would be running if the steal
* had not occurred.
*
* Compensating: Unless there are already enough live threads, method tryCompensate() may create
* or re-activate a spare thread to compensate for blocked joiners until they unblock.
*
* A third form (implemented in tryRemoveAndExec) amounts to helping a hypothetical compensator:
* If we can readily tell that a possible action of a compensator is to steal and execute the task
* being joined, the joining thread can do so directly, without the need for a compensation
* thread.
*
* The ManagedBlocker extension API can't use helping so relies only on compensation in method
* awaitBlocker.
*
* The algorithm in awaitJoin entails a form of "linear helping". Each worker records (in field
* source) the id of the queue from which it last stole a task. The scan in method awaitJoin uses
* these markers to try to find a worker to help (i.e., steal back a task from and execute it)
* that could hasten completion of the actively joined task. Thus, the joiner executes a task that
* would be on its own local deque if the to-be-joined task had not been stolen. This is a
* conservative variant of the approach described in Wagner & Calder "Leapfrogging: a portable
* technique for implementing efficient futures" SIGPLAN Notices, 1993
* (http://portal.acm.org/citation.cfm?id=155354). It differs mainly in that we only record queue
* ids, not full dependency links. This requires a linear scan of the workQueues array to locate
* stealers, but isolates cost to when it is needed, rather than adding to per-task overhead.
* Searches can fail to locate stealers GC stalls and the like delay recording sources. Further,
* even when accurately identified, stealers might not ever produce a task that the joiner can in
* turn help with. So, compensation is tried upon failure to find tasks to run.
*
* Compensation does not by default aim to keep exactly the target parallelism number of unblocked
* threads running at any given time. Some previous versions of this class employed immediate
* compensations for any blocked join. However, in practice, the vast majority of blockages are
* transient byproducts of GC and other JVM or OS activities that are made worse by replacement.
* Rather than impose arbitrary policies, we allow users to override the default of only adding
* threads upon apparent starvation. The compensation mechanism may also be bounded. Bounds for
* the commonPool (see COMMON_MAX_SPARES) better enable JVMs to cope with programming errors and
* abuse before running out of resources to do so.
*
* Common Pool ===========
*
* The static common pool always exists after static initialization. Since it (or any other
* created pool) need never be used, we minimize initial construction overhead and footprint to
* the setup of about a dozen fields.
*
* When external threads submit to the common pool, they can perform subtask processing (see
* externalHelpComplete and related methods) upon joins. This caller-helps policy makes it
* sensible to set common pool parallelism level to one (or more) less than the total number of
* available cores, or even zero for pure caller-runs. We do not need to record whether external
* submissions are to the common pool -- if not, external help methods return quickly. These
* submitters would otherwise be blocked waiting for completion, so the extra effort (with
* liberally sprinkled task status checks) in inapplicable cases amounts to an odd form of limited
* spin-wait before blocking in ForkJoinTask.join.
*
* As a more appropriate default in managed environments, unless overridden by system properties,
* we use workers of subclass InnocuousForkJoinWorkerThread when there is a SecurityManager
* present. These workers have no permissions set, do not belong to any user-defined ThreadGroup,
* and erase all ThreadLocals after executing any top-level task (see
* WorkQueue.afterTopLevelExec). The associated mechanics (mainly in ForkJoinWorkerThread) may be
* JVM-dependent and must access particular Thread class fields to achieve this effect.
*
* Style notes ===========
*
* Memory ordering relies mainly on Unsafe intrinsics that carry the further responsibility of
* explicitly performing null- and bounds- checks otherwise carried out implicitly by JVMs. This
* can be awkward and ugly, but also reflects the need to control outcomes across the unusual
* cases that arise in very racy code with very few invariants. All fields are read into locals
* before use, and null-checked if they are references. This is usually done in a "C"-like style
* of listing declarations at the heads of methods or blocks, and using inline assignments on
* first encounter. Nearly all explicit checks lead to bypass/return, not exception throws,
* because they may legitimately arise due to cancellation/revocation during shutdown.
*
* There is a lot of representation-level coupling among classes ForkJoinPool,
* ForkJoinWorkerThread, and ForkJoinTask. The fields of WorkQueue maintain data structures
* managed by ForkJoinPool, so are directly accessed. There is little point trying to reduce this,
* since any associated future changes in representations will need to be accompanied by
* algorithmic changes anyway. Several methods intrinsically sprawl because they must accumulate
* sets of consistent reads of fields held in local variables. There are also other coding
* oddities (including several unnecessary-looking hoisted null checks) that help some methods
* perform reasonably even when interpreted (not compiled).
*
* The order of declarations in this file is (with a few exceptions): (1) Static utility functions
* (2) Nested (static) classes (3) Static fields (4) Fields, along with constants used when
* unpacking some of them (5) Internal control methods (6) Callbacks and other support for
* ForkJoinTask methods (7) Exported methods (8) Static block initializing statics in minimally
* dependent order
*/
// Static utilities
/**
* Creates a new ForkJoinWorkerThread. This factory is used unless overridden in ForkJoinPool
* constructors.
*/
public static final ForkJoinWorkerThreadFactory defaultForkJoinWorkerThreadFactory;
// Nested classes
// Bounds
static final int SWIDTH = 16; // width of short
static final int SMASK = 0xffff; // short bits == max index
static final int MAX_CAP = 0x7fff; // max #workers - 1
// Constants shared across ForkJoinPool and WorkQueue
static final int SQMASK = 0x007e; // max 64 (even) slots
// Masks and units for WorkQueue.phase and ctl sp subfield
static final int UNSIGNALLED = 1 << 31; // must be negative
static final int SS_SEQ = 1 << 16; // version count
static final int QLOCK = 1; // must be 1
// Mode bits and sentinels, some also used in WorkQueue id and.source fields
static final int OWNED = 1; // queue has owner thread
static final int FIFO = 1 << 16; // fifo queue or access mode
static final int SHUTDOWN = 1 << 18;
static final int TERMINATED = 1 << 19;
static final int STOP = 1 << 31; // must be negative
static final int QUIET = 1 << 30; // not scanning or working
static final int DORMANT = QUIET | UNSIGNALLED;
/**
* The maximum number of local polls from the same queue before checking others. This is a
* safeguard against infinitely unfair looping under unbounded user task recursion, and must be
* larger than plausible cases of intentional bounded task recursion.
*/
static final int POLL_LIMIT = 1 << 10;
/**
* Permission required for callers of methods that may start or kill threads.
*/
static final RuntimePermission modifyThreadPermission;
/**
* Common (static) pool. Non-null for public use unless a static construction exception, but
* internal usages null-check on use to paranoically avoid potential initialization circularities
* as well as to simplify generated code.
*/
static final ForkJoinPool common;
/**
* Common pool parallelism. To allow simpler use and management when common pool threads are
* disabled, we allow the underlying common.parallelism field to be zero, but in that case still
* report parallelism as 1 to reflect resulting caller-runs mechanics.
*/
static final int COMMON_PARALLELISM;
/**
* Limit on spare thread construction in tryCompensate.
*/
private static final int COMMON_MAX_SPARES;
// static fields (initialized in static initializer below)
/**
* Default idle timeout value (in milliseconds) for the thread triggering quiescence to park
* waiting for new work
*/
private static final long DEFAULT_KEEPALIVE = 60_000L;
/**
* Undershoot tolerance for idle timeouts
*/
private static final long TIMEOUT_SLOP = 20L;
/**
* The default value for COMMON_MAX_SPARES. Overridable using the
* "java.util.concurrent.ForkJoinPool.common.maximumSpares" system property. The default value is
* far in excess of normal requirements, but also far short of MAX_CAP and typical OS thread
* limits, so allows JVMs to catch misuse/abuse before running out of resources needed to do so.
*/
private static final int DEFAULT_COMMON_MAX_SPARES = 256;
/**
* Increment for seed generators. See class ThreadLocal for explanation.
*/
private static final int SEED_INCREMENT = 0x9e3779b9;
// Lower and upper word masks
private static final long SP_MASK = 0xffffffffL;
private static final long UC_MASK = ~SP_MASK;
// Release counts
private static final int RC_SHIFT = 48;
// static configuration constants
private static final long RC_UNIT = 0x0001L << RC_SHIFT;
private static final long RC_MASK = 0xffffL << RC_SHIFT;
// Total counts
private static final int TC_SHIFT = 32;
private static final long TC_UNIT = 0x0001L << TC_SHIFT;
/*
* Bits and masks for field ctl, packed with 4 16 bit subfields: RC: Number of released (unqueued)
* workers minus target parallelism TC: Number of total workers minus target parallelism SS:
* version count and status of top waiting thread ID: poolIndex of top of Treiber stack of waiters
*
* When convenient, we can extract the lower 32 stack top bits (including version bits) as
* sp=(int)ctl. The offsets of counts by the target parallelism and the positionings of fields
* makes it possible to perform the most common checks via sign tests of fields: When ac is
* negative, there are not enough unqueued workers, when tc is negative, there are not enough
* total workers. When sp is non-zero, there are waiting workers. To deal with possibly negative
* fields, we use casts in and out of "short" and/or signed shifts to maintain signedness.
*
* Because it occupies uppermost bits, we can add one release count using getAndAddLong of
* RC_UNIT, rather than CAS, when returning from a blocked join. Other updates entail multiple
* subfields and masking, requiring CAS.
*
* The limits packed in field "bounds" are also offset by the parallelism level to make them
* comparable to the ctl rc and tc fields.
*/
private static final long TC_MASK = 0xffffL << TC_SHIFT;
private static final long ADD_WORKER = 0x0001L << (TC_SHIFT + 15); // sign
// Unsafe mechanics
private static final sun.misc.Unsafe U = UnsafeAccess.unsafe;
private static final long CTL;
private static final long MODE;
private static final int ABASE;
private static final int ASHIFT;
/**
* Sequence number for creating workerNamePrefix.
*/
private static int poolNumberSequence;
static {
try {
CTL = U.objectFieldOffset(ForkJoinPool.class.getDeclaredField("ctl"));
MODE = U.objectFieldOffset(ForkJoinPool.class.getDeclaredField("mode"));
ABASE = U.arrayBaseOffset(ForkJoinTask[].class);
int scale = U.arrayIndexScale(ForkJoinTask[].class);
if ((scale & (scale - 1)) != 0)
throw new ExceptionInInitializerError("array index scale not a power of two");
ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
} catch (Exception e) {
throw new ExceptionInInitializerError(e);
}
// Reduce the risk of rare disastrous classloading in first call to
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
@SuppressWarnings("unused")
Class ensureLoaded = LockSupport.class;
int commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
try {
String p = System.getProperty("java.util.concurrent.ForkJoinPool.common.maximumSpares");
if (p != null)
commonMaxSpares = Integer.parseInt(p);
} catch (Exception ignore) {
}
COMMON_MAX_SPARES = commonMaxSpares;
defaultForkJoinWorkerThreadFactory = new DefaultForkJoinWorkerThreadFactory();
modifyThreadPermission = new RuntimePermission("modifyThread");
common = AccessController.doPrivileged(new PrivilegedAction() {
public ForkJoinPool run() {
return new ForkJoinPool((byte) 0);
}
});
COMMON_PARALLELISM = Math.max(common.mode & SMASK, 1);
}
// Instance fields
final long keepAlive; // milliseconds before dropping if idle
final int bounds; // min, max threads packed as shorts
final String workerNamePrefix; // for worker thread string; sync lock
final ForkJoinWorkerThreadFactory factory;
final UncaughtExceptionHandler ueh; // per-worker UEH
final Predicate saturate;
// Segregate ctl field, For now using padding vs @Contended
volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06, pad07;
volatile long pad08, pad09, pad0a, pad0b, pad0c, pad0d, pad0e, pad0f;
volatile long ctl; // main pool control
volatile long pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17;
volatile long pad18, pad19, pad1a, pad1b, pad1c, pad1d, pad1e;
volatile long stealCount; // collects worker nsteals
int indexSeed; // next worker index
volatile int mode; // parallelism, runstate, queue mode
WorkQueue[] workQueues; // main registry
// Creating, registering and deregistering workers
/**
* Creates a {@code ForkJoinPool} with parallelism equal to
* {@link java.lang.Runtime#availableProcessors}, using defaults for all other parameters (see
* {@link #ForkJoinPool(int, ForkJoinWorkerThreadFactory, Thread.UncaughtExceptionHandler, boolean, int, int, int, Predicate, long, TimeUnit)}).
*
* @throws SecurityException if a security manager exists and the caller is not permitted to
* modify threads because it does not hold
* {@link java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool() {
this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()),
defaultForkJoinWorkerThreadFactory, null, false, 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE,
TimeUnit.MILLISECONDS);
}
/**
* Creates a {@code ForkJoinPool} with the indicated parallelism level, using defaults for all
* other parameters (see
* {@link #ForkJoinPool(int, ForkJoinWorkerThreadFactory, Thread.UncaughtExceptionHandler, boolean, int, int, int, Predicate, long, TimeUnit)}).
*
* @param parallelism the parallelism level
* @throws IllegalArgumentException if parallelism less than or equal to zero, or greater than
* implementation limit
* @throws SecurityException if a security manager exists and the caller is not permitted to
* modify threads because it does not hold
* {@link java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool(int parallelism) {
this(parallelism, defaultForkJoinWorkerThreadFactory, null, false, 0, MAX_CAP, 1, null,
DEFAULT_KEEPALIVE, TimeUnit.MILLISECONDS);
}
/**
* Creates a {@code ForkJoinPool} with the given parameters (using defaults for others -- see
* {@link #ForkJoinPool(int, ForkJoinWorkerThreadFactory, Thread.UncaughtExceptionHandler, boolean, int, int, int, Predicate, long, TimeUnit)}).
*
* @param parallelism the parallelism level. For default value, use
* {@link java.lang.Runtime#availableProcessors}.
* @param factory the factory for creating new threads. For default value, use
* {@link #defaultForkJoinWorkerThreadFactory}.
* @param handler the handler for internal worker threads that terminate due to unrecoverable
* errors encountered while executing tasks. For default value, use {@code null}.
* @param asyncMode if true, establishes local first-in-first-out scheduling mode for forked tasks
* that are never joined. This mode may be more appropriate than default locally
* stack-based mode in applications in which worker threads only process event-style
* asynchronous tasks. For default value, use {@code false}.
* @throws IllegalArgumentException if parallelism less than or equal to zero, or greater than
* implementation limit
* @throws NullPointerException if the factory is null
* @throws SecurityException if a security manager exists and the caller is not permitted to
* modify threads because it does not hold
* {@link java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory,
UncaughtExceptionHandler handler, boolean asyncMode) {
this(parallelism, factory, handler, asyncMode, 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE,
TimeUnit.MILLISECONDS);
}
/**
* Creates a {@code ForkJoinPool} with the given parameters.
*
* @param parallelism the parallelism level. For default value, use
* {@link java.lang.Runtime#availableProcessors}.
*
* @param factory the factory for creating new threads. For default value, use
* {@link #defaultForkJoinWorkerThreadFactory}.
*
* @param handler the handler for internal worker threads that terminate due to unrecoverable
* errors encountered while executing tasks. For default value, use {@code null}.
*
* @param asyncMode if true, establishes local first-in-first-out scheduling mode for forked tasks
* that are never joined. This mode may be more appropriate than default locally
* stack-based mode in applications in which worker threads only process event-style
* asynchronous tasks. For default value, use {@code
* false}.
*
* @param corePoolSize the number of threads to keep in the pool (unless timed out after an
* elapsed keep-alive). Normally (and by default) this is the same value as the parallelism
* level, but may be set to a larger value to reduce dynamic overhead if tasks regularly
* block. Using a smaller value (for example {@code 0}) has the same effect as the default.
*
* @param maximumPoolSize the maximum number of threads allowed. When the maximum is reached,
* attempts to replace blocked threads fail. (However, because creation and termination of
* different threads may overlap, and may be managed by the given thread factory, this
* value may be transiently exceeded.) To arrange the same value as is used by default for
* the common pool, use {@code 256} plus the {@code parallelism} level. (By default, the
* common pool allows a maximum of 256 spare threads.) Using a value (for example {@code
* Integer.MAX_VALUE}) larger than the implementation's total thread limit has the same effect as
* using this limit (which is the default).
*
* @param minimumRunnable the minimum allowed number of core threads not blocked by a join or
* {@link ManagedBlocker}. To ensure progress, when too few unblocked threads exist and
* unexecuted tasks may exist, new threads are constructed, up to the given
* maximumPoolSize. For the default value, use {@code
* 1} , that ensures liveness. A larger value might improve throughput in the presence of
* blocked activities, but might not, due to increased overhead. A value of zero may be
* acceptable when submitted tasks cannot have dependencies requiring additional threads.
*
* @param saturate if non-null, a predicate invoked upon attempts to create more than the maximum
* total allowed threads. By default, when a thread is about to block on a join or
* {@link ManagedBlocker}, but cannot be replaced because the maximumPoolSize would be
* exceeded, a {@link RejectedExecutionException} is thrown. But if this predicate returns
* {@code true}, then no exception is thrown, so the pool continues to operate with fewer
* than the target number of runnable threads, which might not ensure progress.
*
* @param keepAliveTime the elapsed time since last use before a thread is terminated (and then
* later replaced if needed). For the default value, use {@code 60, TimeUnit.SECONDS}.
*
* @param unit the time unit for the {@code keepAliveTime} argument
*
* @throws IllegalArgumentException if parallelism is less than or equal to zero, or is greater
* than implementation limit, or if maximumPoolSize is less than parallelism, of if the
* keepAliveTime is less than or equal to zero.
* @throws NullPointerException if the factory is null
* @throws SecurityException if a security manager exists and the caller is not permitted to
* modify threads because it does not hold
* {@link java.lang.RuntimePermission}{@code ("modifyThread")}
* @since 9
*/
public ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory,
UncaughtExceptionHandler handler, boolean asyncMode, int corePoolSize, int maximumPoolSize,
int minimumRunnable, Predicate saturate, long keepAliveTime,
TimeUnit unit) {
// check, encode, pack parameters
if (parallelism <= 0 || parallelism > MAX_CAP || maximumPoolSize < parallelism
|| keepAliveTime <= 0L)
throw new IllegalArgumentException();
Objects.requireNonNull(factory);
long ms = Math.max(unit.toMillis(keepAliveTime), TIMEOUT_SLOP);
int corep = Math.min(Math.max(corePoolSize, parallelism), MAX_CAP);
long c = ((((long) (-corep) << TC_SHIFT) & TC_MASK)
| (((long) (-parallelism) << RC_SHIFT) & RC_MASK));
int m = parallelism | (asyncMode ? FIFO : 0);
int maxSpares = Math.min(maximumPoolSize, MAX_CAP) - parallelism;
int minAvail = Math.min(Math.max(minimumRunnable, 0), MAX_CAP);
int b = ((minAvail - parallelism) & SMASK) | (maxSpares << SWIDTH);
int n = (parallelism > 1) ? parallelism - 1 : 1; // at least 2 slots
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
n = (n + 1) << 1; // power of two, including space for submission queues
this.workerNamePrefix = "ForkJoinPool-" + nextPoolId() + "-worker-";
this.workQueues = new WorkQueue[n];
this.factory = factory;
this.ueh = handler;
this.saturate = saturate;
this.keepAlive = ms;
this.bounds = b;
this.mode = m;
this.ctl = c;
checkPermission();
}
/**
* Constructor for common pool using parameters possibly overridden by system properties
*/
private ForkJoinPool(byte forCommonPoolOnly) {
int parallelism = -1;
ForkJoinWorkerThreadFactory fac = null;
UncaughtExceptionHandler handler = null;
try { // ignore exceptions in accessing/parsing properties
String pp = System.getProperty("java.util.concurrent.ForkJoinPool.common.parallelism");
if (pp != null)
parallelism = Integer.parseInt(pp);
fac = (ForkJoinWorkerThreadFactory) newInstanceFromSystemProperty(
"java.util.concurrent.ForkJoinPool.common.threadFactory");
handler = (UncaughtExceptionHandler) newInstanceFromSystemProperty(
"java.util.concurrent.ForkJoinPool.common.exceptionHandler");
} catch (Exception ignore) {
}
if (fac == null) {
if (System.getSecurityManager() == null) {
fac = defaultForkJoinWorkerThreadFactory;
} else { // use security-managed default
fac = new InnocuousForkJoinWorkerThreadFactory();
}
}
if (parallelism < 0 && // default 1 less than #cores
(parallelism = Runtime.getRuntime().availableProcessors() - 1) <= 0)
parallelism = 1;
if (parallelism > MAX_CAP)
parallelism = MAX_CAP;
long c = ((((long) (-parallelism) << TC_SHIFT) & TC_MASK)
| (((long) (-parallelism) << RC_SHIFT) & RC_MASK));
int b = ((1 - parallelism) & SMASK) | (COMMON_MAX_SPARES << SWIDTH);
int n = (parallelism > 1) ? parallelism - 1 : 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
n = (n + 1) << 1;
this.workerNamePrefix = "ForkJoinPool.commonPool-worker-";
this.workQueues = new WorkQueue[n];
this.factory = fac;
this.ueh = handler;
this.saturate = null;
this.keepAlive = DEFAULT_KEEPALIVE;
this.bounds = b;
this.mode = parallelism;
this.ctl = c;
}
/**
* If there is a security manager, makes sure caller has permission to modify threads.
*/
private static void checkPermission() {
SecurityManager security = System.getSecurityManager();
if (security != null)
security.checkPermission(modifyThreadPermission);
}
static AccessControlContext contextWithPermissions(Permission... perms) {
Permissions permissions = new Permissions();
for (Permission perm : perms)
permissions.add(perm);
return new AccessControlContext(
new ProtectionDomain[] {new ProtectionDomain(null, permissions)});
}
/**
* Returns the next sequence number. We don't expect this to ever contend, so use simple builtin
* sync.
*/
private static final synchronized int nextPoolId() {
return ++poolNumberSequence;
}
/**
* Returns common pool queue for an external thread.
*/
static WorkQueue commonSubmitterQueue() {
ForkJoinPool p = common;
int r = TLRandom.getProbe();
WorkQueue[] ws;
int n;
return (p != null && (ws = p.workQueues) != null && (n = ws.length) > 0)
? ws[(n - 1) & r & SQMASK]
: null;
}
/**
* Returns a cheap heuristic guide for task partitioning when programmers, frameworks, tools, or
* languages have little or no idea about task granularity. In essence, by offering this method,
* we ask users only about tradeoffs in overhead vs expected throughput and its variance, rather
* than how finely to partition tasks.
*
* In a steady state strict (tree-structured) computation, each thread makes available for
* stealing enough tasks for other threads to remain active. Inductively, if all threads play by
* the same rules, each thread should make available only a constant number of tasks.
*
* The minimum useful constant is just 1. But using a value of 1 would require immediate
* replenishment upon each steal to maintain enough tasks, which is infeasible. Further,
* partitionings/granularities of offered tasks should minimize steal rates, which in general
* means that threads nearer the top of computation tree should generate more than those nearer
* the bottom. In perfect steady state, each thread is at approximately the same level of
* computation tree. However, producing extra tasks amortizes the uncertainty of progress and
* diffusion assumptions.
*
* So, users will want to use values larger (but not much larger) than 1 to both smooth over
* transient shortages and hedge against uneven progress; as traded off against the cost of extra
* task overhead. We leave the user to pick a threshold value to compare with the results of this
* call to guide decisions, but recommend values such as 3.
*
* When all threads are active, it is on average OK to estimate surplus strictly locally. In
* steady-state, if one thread is maintaining say 2 surplus tasks, then so are others. So we can
* just use estimated queue length. However, this strategy alone leads to serious mis-estimates in
* some non-steady-state conditions (ramp-up, ramp-down, other stalls). We can detect many of
* these by further considering the number of "idle" threads, that are known to have zero queued
* tasks, so compensate by a factor of (#idle/#active) threads.
*/
static int getSurplusQueuedTaskCount() {
Thread t;
ForkJoinWorkerThread wt;
ForkJoinPool pool;
WorkQueue q;
if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
&& (pool = (wt = (ForkJoinWorkerThread) t).pool) != null && (q = wt.workQueue) != null) {
int p = pool.mode & SMASK;
int a = p + (int) (pool.ctl >> RC_SHIFT);
int n = q.top - q.base;
return n
- (a > (p >>>= 1) ? 0 : a > (p >>>= 1) ? 1 : a > (p >>>= 1) ? 2 : a > (p >>>= 1) ? 4 : 8);
}
return 0;
}
private static Object newInstanceFromSystemProperty(String property) throws Exception {
String className = System.getProperty(property);
return (className == null) ? null
: ClassLoader.getSystemClassLoader().loadClass(className).getConstructor().newInstance();
}
// External operations
/**
* Returns the common pool instance. This pool is statically constructed; its run state is
* unaffected by attempts to {@link #shutdown} or {@link #shutdownNow}. However this pool and any
* ongoing processing are automatically terminated upon program {@link System#exit}. Any program
* that relies on asynchronous task processing to complete before program termination should
* invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence}, before exit.
*
* @return the common pool instance
* @since 1.8
*/
public static ForkJoinPool commonPool() {
// assert common != null : "static init error";
return common;
}
/**
* Returns the targeted parallelism level of the common pool.
*
* @return the targeted parallelism level of the common pool
* @since 1.8
*/
public static int getCommonPoolParallelism() {
return COMMON_PARALLELISM;
}
/**
* Waits and/or attempts to assist performing tasks indefinitely until the {@link #commonPool()}
* {@link #isQuiescent}.
*/
static void quiesceCommonPool() {
common.awaitQuiescence(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
}
/**
* Runs the given possibly blocking task. When {@linkplain ForkJoinTask#inForkJoinPool() running
* in a ForkJoinPool}, this method possibly arranges for a spare thread to be activated if
* necessary to ensure sufficient parallelism while the current thread is blocked in
* {@link ManagedBlocker#block blocker.block()}.
*
*
* This method repeatedly calls {@code blocker.isReleasable()} and {@code blocker.block()} until
* either method returns {@code true}. Every call to {@code blocker.block()} is preceded by a call
* to {@code blocker.isReleasable()} that returned {@code false}.
*
*
* If not running in a ForkJoinPool, this method is behaviorally equivalent to
*
*
* {@code
* while (!blocker.isReleasable())
* if (blocker.block())
* break;}
*
*
* If running in a ForkJoinPool, the pool may first be expanded to ensure sufficient parallelism
* available during the call to {@code blocker.block()}.
*
* @param blocker the blocker task
* @throws InterruptedException if {@code blocker.block()} did so
*/
public static void managedBlock(ManagedBlocker blocker) throws InterruptedException {
ForkJoinPool p;
ForkJoinWorkerThread wt;
WorkQueue w;
Thread t = Thread.currentThread();
if ((t instanceof ForkJoinWorkerThread) && (p = (wt = (ForkJoinWorkerThread) t).pool) != null
&& (w = wt.workQueue) != null) {
int block;
while (!blocker.isReleasable()) {
if ((block = p.tryCompensate(w)) != 0) {
try {
do {
} while (!blocker.isReleasable() && !blocker.block());
} finally {
U.getAndAddLong(p, CTL, (block > 0) ? RC_UNIT : 0L);
}
break;
}
}
} else {
do {
} while (!blocker.isReleasable() && !blocker.block());
}
}
/**
* If the given executor is a ForkJoinPool, poll and execute AsynchronousCompletionTasks from
* worker's queue until none are available or blocker is released.
*/
static void helpAsyncBlocker(Executor e, ManagedBlocker blocker) {
if (blocker != null && (e instanceof ForkJoinPool)) {
WorkQueue w;
ForkJoinWorkerThread wt;
WorkQueue[] ws;
int r, n;
ForkJoinPool p = (ForkJoinPool) e;
Thread thread = Thread.currentThread();
if (thread instanceof ForkJoinWorkerThread && (wt = (ForkJoinWorkerThread) thread).pool == p)
w = wt.workQueue;
else if ((r = TLRandom.getProbe()) != 0 && (ws = p.workQueues) != null && (n = ws.length) > 0)
w = ws[(n - 1) & r & SQMASK];
else
w = null;
if (w != null) {
for (;;) {
int b = w.base, s = w.top, d, al;
ForkJoinTask[] a;
if ((a = w.array) != null && (d = b - s) < 0 && (al = a.length) > 0) {
int index = (al - 1) & b;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getObjectVolatile(a, offset);
if (blocker.isReleasable())
break;
else if (b++ == w.base) {
if (t == null) {
if (d == -1)
break;
} else if (!(t instanceof CompletableFuture.AsynchronousCompletionTask))
break;
else if (U.compareAndSwapObject(a, offset, t, null)) {
w.base = b;
t.doExec();
}
}
} else
break;
}
}
}
}
/**
* Tries to construct and start one worker. Assumes that total count has already been incremented
* as a reservation. Invokes deregisterWorker on any failure.
*
* @return true if successful
*/
private boolean createWorker() {
ForkJoinWorkerThreadFactory fac = factory;
Throwable ex = null;
ForkJoinWorkerThread wt = null;
try {
if (fac != null && (wt = fac.newThread(this)) != null) {
wt.start();
return true;
}
} catch (Throwable rex) {
ex = rex;
}
deregisterWorker(wt, ex);
return false;
}
/**
* Tries to add one worker, incrementing ctl counts before doing so, relying on createWorker to
* back out on failure.
*
* @param c incoming ctl value, with total count negative and no idle workers. On CAS failure, c
* is refreshed and retried if this holds (otherwise, a new worker is not needed).
*/
private void tryAddWorker(long c) {
do {
long nc = ((RC_MASK & (c + RC_UNIT)) | (TC_MASK & (c + TC_UNIT)));
if (ctl == c && U.compareAndSwapLong(this, CTL, c, nc)) {
createWorker();
break;
}
} while (((c = ctl) & ADD_WORKER) != 0L && (int) c == 0);
}
// Termination
/**
* Callback from ForkJoinWorkerThread constructor to establish and record its WorkQueue.
*
* @param wt the worker thread
* @return the worker's queue
*/
final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
UncaughtExceptionHandler handler;
wt.setDaemon(true); // configure thread
if ((handler = ueh) != null)
wt.setUncaughtExceptionHandler(handler);
WorkQueue w = new WorkQueue(this, wt);
int tid = 0; // for thread name
int fifo = mode & FIFO;
String prefix = workerNamePrefix;
if (prefix != null) {
synchronized (prefix) {
WorkQueue[] ws = workQueues;
int n;
int s = indexSeed += SEED_INCREMENT;
if (ws != null && (n = ws.length) > 1) {
int m = n - 1;
tid = s & m;
int i = m & ((s << 1) | 1); // odd-numbered indices
for (int probes = n >>> 1;;) { // find empty slot
WorkQueue q;
if ((q = ws[i]) == null || q.phase == QUIET)
break;
else if (--probes == 0) {
i = n | 1; // resize below
break;
} else
i = (i + 2) & m;
}
int id = i | fifo | (s & ~(SMASK | FIFO | DORMANT));
w.phase = w.id = id; // now publishable
if (i < n)
ws[i] = w;
else { // expand array
int an = n << 1;
WorkQueue[] as = new WorkQueue[an];
as[i] = w;
int am = an - 1;
for (int j = 0; j < n; ++j) {
WorkQueue v; // copy external queue
if ((v = ws[j]) != null) // position may change
as[v.id & am & SQMASK] = v;
if (++j >= n)
break;
as[j] = ws[j]; // copy worker
}
workQueues = as;
}
}
}
wt.setName(prefix.concat(Integer.toString(tid)));
}
return w;
}
// Exported methods
// Constructors
/**
* Final callback from terminating worker, as well as upon failure to construct or start a worker.
* Removes record of worker from array, and adjusts counts. If pool is shutting down, tries to
* complete termination.
*
* @param wt the worker thread, or null if construction failed
* @param ex the exception causing failure, or null if none
*/
final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
WorkQueue w = null;
int phase = 0;
if (wt != null && (w = wt.workQueue) != null) {
Object lock = workerNamePrefix;
long ns = (long) w.nsteals & 0xffffffffL;
int idx = w.id & SMASK;
if (lock != null) {
WorkQueue[] ws; // remove index from array
synchronized (lock) {
if ((ws = workQueues) != null && ws.length > idx && ws[idx] == w)
ws[idx] = null;
stealCount += ns;
}
}
phase = w.phase;
}
if (phase != QUIET) { // else pre-adjusted
long c; // decrement counts
do {
} while (!U.compareAndSwapLong(this, CTL, c = ctl,
((RC_MASK & (c - RC_UNIT)) | (TC_MASK & (c - TC_UNIT)) | (SP_MASK & c))));
}
if (w != null)
w.cancelAll(); // cancel remaining tasks
if (!tryTerminate(false, false) && // possibly replace worker
w != null && w.array != null) // avoid repeated failures
signalWork();
if (ex == null) // help clean on way out
ForkJoinTask.helpExpungeStaleExceptions();
else // rethrow
ForkJoinTask.rethrow(ex);
}
/**
* Tries to create or release a worker if too few are running.
*/
final void signalWork() {
for (;;) {
long c;
int sp;
WorkQueue[] ws;
int i;
WorkQueue v;
if ((c = ctl) >= 0L) // enough workers
break;
else if ((sp = (int) c) == 0) { // no idle workers
if ((c & ADD_WORKER) != 0L) // too few workers
tryAddWorker(c);
break;
} else if ((ws = workQueues) == null)
break; // unstarted/terminated
else if (ws.length <= (i = sp & SMASK))
break; // terminated
else if ((v = ws[i]) == null)
break; // terminating
else {
int np = sp & ~UNSIGNALLED;
int vp = v.phase;
long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + RC_UNIT));
Thread vt = v.owner;
if (sp == vp && U.compareAndSwapLong(this, CTL, c, nc)) {
v.phase = np;
if (v.source < 0)
LockSupport.unpark(vt);
break;
}
}
}
}
/**
* Tries to decrement counts (sometimes implicitly) and possibly arrange for a compensating worker
* in preparation for blocking: If not all core workers yet exist, creates one, else if any are
* unreleased (possibly including caller) releases one, else if fewer than the minimum allowed
* number of workers running, checks to see that they are all active, and if so creates an extra
* worker unless over maximum limit and policy is to saturate. Most of these steps can fail due to
* interference, in which case 0 is returned so caller will retry. A negative return value
* indicates that the caller doesn't need to re-adjust counts when later unblocked.
*
* @return 1: block then adjust, -1: block without adjust, 0 : retry
*/
private int tryCompensate(WorkQueue w) {
int t, n, sp;
long c = ctl;
WorkQueue[] ws = workQueues;
if ((t = (short) (c >>> TC_SHIFT)) >= 0) {
if (ws == null || (n = ws.length) <= 0 || w == null)
return 0; // disabled
else if ((sp = (int) c) != 0) { // replace or release
WorkQueue v = ws[sp & (n - 1)];
int wp = w.phase;
long uc = UC_MASK & ((wp < 0) ? c + RC_UNIT : c);
int np = sp & ~UNSIGNALLED;
if (v != null) {
int vp = v.phase;
Thread vt = v.owner;
long nc = ((long) v.stackPred & SP_MASK) | uc;
if (vp == sp && U.compareAndSwapLong(this, CTL, c, nc)) {
v.phase = np;
if (v.source < 0)
LockSupport.unpark(vt);
return (wp < 0) ? -1 : 1;
}
}
return 0;
} else if ((int) (c >> RC_SHIFT) - // reduce parallelism
(short) (bounds & SMASK) > 0) {
long nc = ((RC_MASK & (c - RC_UNIT)) | (~RC_MASK & c));
return U.compareAndSwapLong(this, CTL, c, nc) ? 1 : 0;
} else { // validate
int md = mode, pc = md & SMASK, tc = pc + t, bc = 0;
boolean unstable = false;
for (int i = 1; i < n; i += 2) {
WorkQueue q;
Thread wt;
Thread.State ts;
if ((q = ws[i]) != null) {
if (q.source == 0) {
unstable = true;
break;
} else {
--tc;
if ((wt = q.owner) != null
&& ((ts = wt.getState()) == Thread.State.BLOCKED || ts == Thread.State.WAITING))
++bc; // worker is blocking
}
}
}
if (unstable || tc != 0 || ctl != c)
return 0; // inconsistent
else if (t + pc >= MAX_CAP || t >= (bounds >>> SWIDTH)) {
Predicate sat;
if ((sat = saturate) != null && sat.test(this))
return -1;
else if (bc < pc) { // lagging
Thread.yield(); // for retry spins
return 0;
} else
throw new RejectedExecutionException("Thread limit exceeded replacing blocked worker");
}
}
}
long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK); // expand pool
return U.compareAndSwapLong(this, CTL, c, nc) && createWorker() ? 1 : 0;
}
/**
* Top-level runloop for workers, called by ForkJoinWorkerThread.run. See above for explanation.
*/
final void runWorker(WorkQueue w) {
WorkQueue[] ws;
w.growArray(); // allocate queue
int r = w.id ^ TLRandom.nextSecondarySeed();
if (r == 0) // initial nonzero seed
r = 1;
int lastSignalId = 0; // avoid unneeded signals
while ((ws = workQueues) != null) {
boolean nonempty = false; // scan
for (int n = ws.length, j = n, m = n - 1; j > 0; --j) {
WorkQueue q;
int i, b, al;
ForkJoinTask[] a;
if ((i = r & m) >= 0 && i < n && // always true
(q = ws[i]) != null && (b = q.base) - q.top < 0 && (a = q.array) != null
&& (al = a.length) > 0) {
int qid = q.id; // (never zero)
int index = (al - 1) & b;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getObjectVolatile(a, offset);
if (t != null && b++ == q.base && U.compareAndSwapObject(a, offset, t, null)) {
if ((q.base = b) - q.top < 0 && qid != lastSignalId)
signalWork(); // propagate signal
w.source = lastSignalId = qid;
t.doExec();
if ((w.id & FIFO) != 0) // run remaining locals
w.localPollAndExec(POLL_LIMIT);
else
w.localPopAndExec(POLL_LIMIT);
ForkJoinWorkerThread thread = w.owner;
++w.nsteals;
w.source = 0; // now idle
if (thread != null)
thread.afterTopLevelExec();
}
nonempty = true;
} else if (nonempty)
break;
else
++r;
}
if (nonempty) { // move (xorshift)
r ^= r << 13;
r ^= r >>> 17;
r ^= r << 5;
} else {
int phase;
lastSignalId = 0; // clear for next scan
if ((phase = w.phase) >= 0) { // enqueue
int np = w.phase = (phase + SS_SEQ) | UNSIGNALLED;
long c, nc;
do {
w.stackPred = (int) (c = ctl);
nc = ((c - RC_UNIT) & UC_MASK) | (SP_MASK & np);
} while (!U.compareAndSwapLong(this, CTL, c, nc));
} else { // already queued
int pred = w.stackPred;
w.source = DORMANT; // enable signal
for (int steps = 0;;) {
int md, rc;
long c;
if (w.phase >= 0) {
w.source = 0;
break;
} else if ((md = mode) < 0) // shutting down
return;
else if ((rc = ((md & SMASK) + // possibly quiescent
(int) ((c = ctl) >> RC_SHIFT))) <= 0 && (md & SHUTDOWN) != 0
&& tryTerminate(false, false))
return; // help terminate
else if ((++steps & 1) == 0)
Thread.interrupted(); // clear between parks
else if (rc <= 0 && pred != 0 && phase == (int) c) {
long d = keepAlive + System.currentTimeMillis();
LockSupport.parkUntil(this, d);
if (ctl == c && d - System.currentTimeMillis() <= TIMEOUT_SLOP) {
long nc = ((UC_MASK & (c - TC_UNIT)) | (SP_MASK & pred));
if (U.compareAndSwapLong(this, CTL, c, nc)) {
w.phase = QUIET;
return; // drop on timeout
}
}
} else
LockSupport.park(this);
}
}
}
}
}
/**
* Helps and/or blocks until the given task is done or timeout. First tries locally helping, then
* scans other queues for a task produced by one of w's stealers; compensating and blocking if
* none are found (rescanning if tryCompensate fails).
*
* @param w caller
* @param task the task
* @param deadline for timed waits, if nonzero
* @return task status on exit
*/
final int awaitJoin(WorkQueue w, ForkJoinTask task, long deadline) {
int s = 0;
if (w != null && task != null && (!(task instanceof CountedCompleter)
|| (s = w.localHelpCC((CountedCompleter) task, 0)) >= 0)) {
w.tryRemoveAndExec(task);
int src = w.source, id = w.id;
s = task.status;
while (s >= 0) {
WorkQueue[] ws;
boolean nonempty = false;
int r = TLRandom.nextSecondarySeed() | 1; // odd indices
if ((ws = workQueues) != null) { // scan for matching id
for (int n = ws.length, m = n - 1, j = -n; j < n; j += 2) {
WorkQueue q;
int i, b, al;
ForkJoinTask[] a;
if ((i = (r + j) & m) >= 0 && i < n && (q = ws[i]) != null && q.source == id
&& (b = q.base) - q.top < 0 && (a = q.array) != null && (al = a.length) > 0) {
int qid = q.id;
int index = (al - 1) & b;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getObjectVolatile(a, offset);
if (t != null && b++ == q.base && id == q.source
&& U.compareAndSwapObject(a, offset, t, null)) {
q.base = b;
w.source = qid;
t.doExec();
w.source = src;
}
nonempty = true;
break;
}
}
}
if ((s = task.status) < 0)
break;
else if (!nonempty) {
long ms, ns;
int block;
if (deadline == 0L)
ms = 0L; // untimed
else if ((ns = deadline - System.nanoTime()) <= 0L)
break; // timeout
else if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) <= 0L)
ms = 1L; // avoid 0 for timed wait
if ((block = tryCompensate(w)) != 0) {
task.internalWait(ms);
U.getAndAddLong(this, CTL, (block > 0) ? RC_UNIT : 0L);
}
s = task.status;
}
}
}
return s;
}
/**
* Runs tasks until {@code isQuiescent()}. Rather than blocking when tasks cannot be found,
* rescans until all others cannot find tasks either.
*/
final void helpQuiescePool(WorkQueue w) {
int prevSrc = w.source, fifo = w.id & FIFO;
for (int source = prevSrc, released = -1;;) { // -1 until known
WorkQueue[] ws;
if (fifo != 0)
w.localPollAndExec(0);
else
w.localPopAndExec(0);
if (released == -1 && w.phase >= 0)
released = 1;
boolean quiet = true, empty = true;
int r = TLRandom.nextSecondarySeed();
if ((ws = workQueues) != null) {
for (int n = ws.length, j = n, m = n - 1; j > 0; --j) {
WorkQueue q;
int i, b, al;
ForkJoinTask[] a;
if ((i = (r - j) & m) >= 0 && i < n && (q = ws[i]) != null) {
if ((b = q.base) - q.top < 0 && (a = q.array) != null && (al = a.length) > 0) {
int qid = q.id;
if (released == 0) { // increment
released = 1;
U.getAndAddLong(this, CTL, RC_UNIT);
}
int index = (al - 1) & b;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getObjectVolatile(a, offset);
if (t != null && b++ == q.base && U.compareAndSwapObject(a, offset, t, null)) {
q.base = b;
w.source = source = q.id;
t.doExec();
w.source = source = prevSrc;
}
quiet = empty = false;
break;
} else if ((q.source & QUIET) == 0)
quiet = false;
}
}
}
if (quiet) {
if (released == 0)
U.getAndAddLong(this, CTL, RC_UNIT);
w.source = prevSrc;
break;
} else if (empty) {
if (source != QUIET)
w.source = source = QUIET;
if (released == 1) { // decrement
released = 0;
U.getAndAddLong(this, CTL, RC_MASK & -RC_UNIT);
}
}
}
}
/**
* Scans for and returns a polled task, if available. Used only for untracked polls.
*
* @param submissionsOnly if true, only scan submission queues
*/
private ForkJoinTask pollScan(boolean submissionsOnly) {
WorkQueue[] ws;
int n;
rescan: while ((mode & STOP) == 0 && (ws = workQueues) != null && (n = ws.length) > 0) {
int m = n - 1;
int r = TLRandom.nextSecondarySeed();
int h = r >>> 16;
int origin, step;
if (submissionsOnly) {
origin = (r & ~1) & m; // even indices and steps
step = (h & ~1) | 2;
} else {
origin = r & m;
step = h | 1;
}
for (int k = origin, oldSum = 0, checkSum = 0;;) {
WorkQueue q;
int b, al;
ForkJoinTask[] a;
if ((q = ws[k]) != null) {
checkSum += b = q.base;
if (b - q.top < 0 && (a = q.array) != null && (al = a.length) > 0) {
int index = (al - 1) & b;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getObjectVolatile(a, offset);
if (t != null && b++ == q.base && U.compareAndSwapObject(a, offset, t, null)) {
q.base = b;
return t;
} else
break; // restart
}
}
if ((k = (k + step) & m) == origin) {
if (oldSum == (oldSum = checkSum))
break rescan;
checkSum = 0;
}
}
}
return null;
}
// Execution methods
/**
* Gets and removes a local or stolen task for the given worker.
*
* @return a task, if available
*/
final ForkJoinTask nextTaskFor(WorkQueue w) {
ForkJoinTask t;
if (w != null && (t = (w.id & FIFO) != 0 ? w.poll() : w.pop()) != null)
return t;
else
return pollScan(false);
}
/**
* Adds the given task to a submission queue at submitter's current queue, creating one if null or
* contended.
*
* @param task the task. Caller must ensure non-null.
*/
final void externalPush(ForkJoinTask task) {
int r; // initialize caller's probe
if ((r = TLRandom.getProbe()) == 0) {
TLRandom.localInit();
r = TLRandom.getProbe();
}
for (;;) {
int md = mode, n;
WorkQueue[] ws = workQueues;
if ((md & SHUTDOWN) != 0 || ws == null || (n = ws.length) <= 0)
throw new RejectedExecutionException();
else {
WorkQueue q;
boolean push = false, grow = false;
if ((q = ws[(n - 1) & r & SQMASK]) == null) {
Object lock = workerNamePrefix;
int qid = (r | QUIET) & ~(FIFO | OWNED);
q = new WorkQueue(this, null);
q.id = qid;
q.source = QUIET;
q.phase = QLOCK; // lock queue
if (lock != null) {
synchronized (lock) { // lock pool to install
int i;
if ((ws = workQueues) != null && (n = ws.length) > 0
&& ws[i = qid & (n - 1) & SQMASK] == null) {
ws[i] = q;
push = grow = true;
}
}
}
} else if (q.tryLockSharedQueue()) {
int b = q.base, s = q.top, al, d;
ForkJoinTask[] a;
if ((a = q.array) != null && (al = a.length) > 0 && al - 1 + (d = b - s) > 0) {
a[(al - 1) & s] = task;
q.top = s + 1; // relaxed writes OK here
q.phase = 0;
if (d < 0 && q.base - s < -1)
break; // no signal needed
} else
grow = true;
push = true;
}
if (push) {
if (grow) {
try {
q.growArray();
int s = q.top, al;
ForkJoinTask[] a;
if ((a = q.array) != null && (al = a.length) > 0) {
a[(al - 1) & s] = task;
q.top = s + 1;
}
} finally {
q.phase = 0;
}
}
signalWork();
break;
} else // move if busy
r = TLRandom.advanceProbe(r);
}
}
}
// AbstractExecutorService methods
/**
* Pushes a possibly-external submission.
*/
private ForkJoinTask externalSubmit(ForkJoinTask task) {
Thread t;
ForkJoinWorkerThread w;
WorkQueue q;
Objects.requireNonNull(task);
if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
&& (w = (ForkJoinWorkerThread) t).pool == this && (q = w.workQueue) != null)
q.push(task);
else
externalPush(task);
return task;
}
/**
* Performs tryUnpush for an external submitter.
*/
final boolean tryExternalUnpush(ForkJoinTask task) {
int r = TLRandom.getProbe();
WorkQueue[] ws;
WorkQueue w;
int n;
return ((ws = workQueues) != null && (n = ws.length) > 0
&& (w = ws[(n - 1) & r & SQMASK]) != null && w.trySharedUnpush(task));
}
/**
* Performs helpComplete for an external submitter.
*/
final int externalHelpComplete(CountedCompleter task, int maxTasks) {
int r = TLRandom.getProbe();
WorkQueue[] ws;
WorkQueue w;
int n;
return ((ws = workQueues) != null && (n = ws.length) > 0
&& (w = ws[(n - 1) & r & SQMASK]) != null) ? w.sharedHelpCC(task, maxTasks) : 0;
}
/**
* Tries to steal and run tasks within the target's computation. The maxTasks argument supports
* external usages; internal calls use zero, allowing unbounded steps (external calls trap
* non-positive values).
*
* @param w caller
* @param maxTasks if non-zero, the maximum number of other tasks to run
* @return task status on exit
*/
final int helpComplete(WorkQueue w, CountedCompleter task, int maxTasks) {
return (w == null) ? 0 : w.localHelpCC(task, maxTasks);
}
/**
* Possibly initiates and/or completes termination.
*
* @param now if true, unconditionally terminate, else only if no work and no active workers
* @param enable if true, terminate when next possible
* @return true if terminating or terminated
*/
private boolean tryTerminate(boolean now, boolean enable) {
int md; // 3 phases: try to set SHUTDOWN, then STOP, then TERMINATED
while (((md = mode) & SHUTDOWN) == 0) {
if (!enable || this == common) // cannot shutdown
return false;
else
U.compareAndSwapInt(this, MODE, md, md | SHUTDOWN);
}
while (((md = mode) & STOP) == 0) { // try to initiate termination
if (!now) { // check if quiescent & empty
for (long oldSum = 0L;;) { // repeat until stable
boolean running = false;
long checkSum = ctl;
WorkQueue[] ws = workQueues;
if ((md & SMASK) + (int) (checkSum >> RC_SHIFT) > 0)
running = true;
else if (ws != null) {
WorkQueue w;
for (int i = 0; i < ws.length; ++i) {
if ((w = ws[i]) != null) {
int s = w.source, p = w.phase;
int d = w.id, b = w.base;
if (b != w.top || ((d & 1) == 1 && (s >= 0 || p >= 0))) {
running = true;
break; // working, scanning, or have work
}
checkSum += (((long) s << 48) + ((long) p << 32) + ((long) b << 16) + (long) d);
}
}
}
if (((md = mode) & STOP) != 0)
break; // already triggered
else if (running)
return false;
else if (workQueues == ws && oldSum == (oldSum = checkSum))
break;
}
}
if ((md & STOP) == 0)
U.compareAndSwapInt(this, MODE, md, md | STOP);
}
while (((md = mode) & TERMINATED) == 0) { // help terminate others
for (long oldSum = 0L;;) { // repeat until stable
WorkQueue[] ws;
WorkQueue w;
long checkSum = ctl;
if ((ws = workQueues) != null) {
for (int i = 0; i < ws.length; ++i) {
if ((w = ws[i]) != null) {
ForkJoinWorkerThread wt = w.owner;
w.cancelAll(); // clear queues
if (wt != null) {
try { // unblock join or park
wt.interrupt();
} catch (Throwable ignore) {
}
}
checkSum += ((long) w.phase << 32) + w.base;
}
}
}
if (((md = mode) & TERMINATED) != 0 || (workQueues == ws && oldSum == (oldSum = checkSum)))
break;
}
if ((md & TERMINATED) != 0)
break;
else if ((md & SMASK) + (short) (ctl >>> TC_SHIFT) > 0)
break;
else if (U.compareAndSwapInt(this, MODE, md, md | TERMINATED)) {
synchronized (this) {
notifyAll(); // for awaitTermination
}
break;
}
}
return true;
}
/**
* Performs the given task, returning its result upon completion. If the computation encounters an
* unchecked Exception or Error, it is rethrown as the outcome of this invocation. Rethrown
* exceptions behave in the same way as regular exceptions, but, when possible, contain stack
* traces (as displayed for example using {@code ex.printStackTrace()}) of both the current thread
* as well as the thread actually encountering the exception; minimally only the latter.
*
* @param task the task
* @param the type of the task's result
* @return the task's result
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be scheduled for execution
*/
public T invoke(ForkJoinTask task) {
externalSubmit(Objects.requireNonNull(task));
return task.join();
}
/**
* Arranges for (asynchronous) execution of the given task.
*
* @param task the task
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be scheduled for execution
*/
public void execute(ForkJoinTask task) {
externalSubmit(task);
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be scheduled for execution
*/
public void execute(Runnable task) {
Objects.requireNonNull(task);
ForkJoinTask job;
if (task instanceof ForkJoinTask) // avoid re-wrap
job = (ForkJoinTask) task;
else
job = new ForkJoinTask.RunnableExecuteAction(task);
externalSubmit(job);
}
/**
* Submits a ForkJoinTask for execution.
*
* @param task the task to submit
* @param the type of the task's result
* @return the task
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be scheduled for execution
*/
public ForkJoinTask submit(ForkJoinTask task) {
return externalSubmit(task);
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be scheduled for execution
*/
public ForkJoinTask submit(Callable task) {
return externalSubmit(new ForkJoinTask.AdaptedCallable(task));
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be scheduled for execution
*/
public ForkJoinTask submit(Runnable task, T result) {
return externalSubmit(new ForkJoinTask.AdaptedRunnable(task, result));
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be scheduled for execution
*/
@SuppressWarnings("unchecked")
public ForkJoinTask submit(Runnable task) {
Objects.requireNonNull(task);
return externalSubmit((task instanceof ForkJoinTask) ? (ForkJoinTask) task // avoid
// re-wrap
: new ForkJoinTask.AdaptedRunnableAction(task));
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws RejectedExecutionException {@inheritDoc}
*/
public List> invokeAll(Collection> tasks) {
// In previous versions of this class, this method constructed
// a task to run ForkJoinTask.invokeAll, but now external
// invocation of multiple tasks is at least as efficient.
ArrayList> futures = new ArrayList<>(tasks.size());
try {
for (Callable t : tasks) {
ForkJoinTask f = new ForkJoinTask.AdaptedCallable(t);
futures.add(f);
externalSubmit(f);
}
for (int i = 0, size = futures.size(); i < size; i++)
((ForkJoinTask) futures.get(i)).quietlyJoin();
return futures;
} catch (Throwable t) {
for (int i = 0, size = futures.size(); i < size; i++)
futures.get(i).cancel(false);
throw t;
}
}
/**
* Returns the factory used for constructing new workers.
*
* @return the factory used for constructing new workers
*/
public ForkJoinWorkerThreadFactory getFactory() {
return factory;
}
/**
* Returns the handler for internal worker threads that terminate due to unrecoverable errors
* encountered while executing tasks.
*
* @return the handler, or {@code null} if none
*/
public UncaughtExceptionHandler getUncaughtExceptionHandler() {
return ueh;
}
/**
* Returns the targeted parallelism level of this pool.
*
* @return the targeted parallelism level of this pool
*/
public int getParallelism() {
int par = mode & SMASK;
return (par > 0) ? par : 1;
}
/**
* Returns the number of worker threads that have started but not yet terminated. The result
* returned by this method may differ from {@link #getParallelism} when threads are created to
* maintain parallelism when others are cooperatively blocked.
*
* @return the number of worker threads
*/
public int getPoolSize() {
return ((mode & SMASK) + (short) (ctl >>> TC_SHIFT));
}
/**
* Returns {@code true} if this pool uses local first-in-first-out scheduling mode for forked
* tasks that are never joined.
*
* @return {@code true} if this pool uses async mode
*/
public boolean getAsyncMode() {
return (mode & FIFO) != 0;
}
/**
* Returns an estimate of the number of worker threads that are not blocked waiting to join tasks
* or for other managed synchronization. This method may overestimate the number of running
* threads.
*
* @return the number of worker threads
*/
public int getRunningThreadCount() {
int rc = 0;
WorkQueue[] ws;
WorkQueue w;
if ((ws = workQueues) != null) {
for (int i = 1; i < ws.length; i += 2) {
if ((w = ws[i]) != null && w.isApparentlyUnblocked())
++rc;
}
}
return rc;
}
/**
* Returns an estimate of the number of threads that are currently stealing or executing tasks.
* This method may overestimate the number of active threads.
*
* @return the number of active threads
*/
public int getActiveThreadCount() {
int r = (mode & SMASK) + (int) (ctl >> RC_SHIFT);
return (r <= 0) ? 0 : r; // suppress momentarily negative values
}
/**
* Returns {@code true} if all worker threads are currently idle. An idle worker is one that
* cannot obtain a task to execute because none are available to steal from other threads, and
* there are no pending submissions to the pool. This method is conservative; it might not return
* {@code true} immediately upon idleness of all threads, but will eventually become true if
* threads remain inactive.
*
* @return {@code true} if all threads are currently idle
*/
public boolean isQuiescent() {
for (;;) {
long c = ctl;
int md = mode, pc = md & SMASK;
int tc = pc + (short) (c >>> TC_SHIFT);
int rc = pc + (int) (c >> RC_SHIFT);
if ((md & (STOP | TERMINATED)) != 0)
return true;
else if (rc > 0)
return false;
else {
WorkQueue[] ws;
WorkQueue v;
if ((ws = workQueues) != null) {
for (int i = 1; i < ws.length; i += 2) {
if ((v = ws[i]) != null) {
if ((v.source & QUIET) == 0)
return false;
--tc;
}
}
}
if (tc == 0 && ctl == c)
return true;
}
}
}
/**
* Returns an estimate of the total number of tasks stolen from one thread's work queue by
* another. The reported value underestimates the actual total number of steals when the pool is
* not quiescent. This value may be useful for monitoring and tuning fork/join programs: in
* general, steal counts should be high enough to keep threads busy, but low enough to avoid
* overhead and contention across threads.
*
* @return the number of steals
*/
public long getStealCount() {
long count = stealCount;
WorkQueue[] ws;
WorkQueue w;
if ((ws = workQueues) != null) {
for (int i = 1; i < ws.length; i += 2) {
if ((w = ws[i]) != null)
count += (long) w.nsteals & 0xffffffffL;
}
}
return count;
}
/**
* Returns an estimate of the total number of tasks currently held in queues by worker threads
* (but not including tasks submitted to the pool that have not begun executing). This value is
* only an approximation, obtained by iterating across all threads in the pool. This method may be
* useful for tuning task granularities.
*
* @return the number of queued tasks
*/
public long getQueuedTaskCount() {
long count = 0;
WorkQueue[] ws;
WorkQueue w;
if ((ws = workQueues) != null) {
for (int i = 1; i < ws.length; i += 2) {
if ((w = ws[i]) != null)
count += w.queueSize();
}
}
return count;
}
/**
* Returns an estimate of the number of tasks submitted to this pool that have not yet begun
* executing. This method may take time proportional to the number of submissions.
*
* @return the number of queued submissions
*/
public int getQueuedSubmissionCount() {
int count = 0;
WorkQueue[] ws;
WorkQueue w;
if ((ws = workQueues) != null) {
for (int i = 0; i < ws.length; i += 2) {
if ((w = ws[i]) != null)
count += w.queueSize();
}
}
return count;
}
/**
* Returns {@code true} if there are any tasks submitted to this pool that have not yet begun
* executing.
*
* @return {@code true} if there are any queued submissions
*/
public boolean hasQueuedSubmissions() {
WorkQueue[] ws;
WorkQueue w;
if ((ws = workQueues) != null) {
for (int i = 0; i < ws.length; i += 2) {
if ((w = ws[i]) != null && !w.isEmpty())
return true;
}
}
return false;
}
/**
* Removes and returns the next unexecuted submission if one is available. This method may be
* useful in extensions to this class that re-assign work in systems with multiple pools.
*
* @return the next submission, or {@code null} if none
*/
protected ForkJoinTask pollSubmission() {
return pollScan(true);
}
/**
* Removes all available unexecuted submitted and forked tasks from scheduling queues and adds
* them to the given collection, without altering their execution status. These may include
* artificially generated or wrapped tasks. This method is designed to be invoked only when the
* pool is known to be quiescent. Invocations at other times may not remove all tasks. A failure
* encountered while attempting to add elements to collection {@code c} may result in elements
* being in neither, either or both collections when the associated exception is thrown. The
* behavior of this operation is undefined if the specified collection is modified while the
* operation is in progress.
*
* @param c the collection to transfer elements into
* @return the number of elements transferred
*/
protected int drainTasksTo(Collection> c) {
int count = 0;
WorkQueue[] ws;
WorkQueue w;
ForkJoinTask t;
if ((ws = workQueues) != null) {
for (int i = 0; i < ws.length; ++i) {
if ((w = ws[i]) != null) {
while ((t = w.poll()) != null) {
c.add(t);
++count;
}
}
}
}
return count;
}
/**
* Returns a string identifying this pool, as well as its state, including indications of run
* state, parallelism level, and worker and task counts.
*
* @return a string identifying this pool, as well as its state
*/
public String toString() {
// Use a single pass through workQueues to collect counts
long qt = 0L, qs = 0L;
int rc = 0;
long st = stealCount;
WorkQueue[] ws;
WorkQueue w;
if ((ws = workQueues) != null) {
for (int i = 0; i < ws.length; ++i) {
if ((w = ws[i]) != null) {
int size = w.queueSize();
if ((i & 1) == 0)
qs += size;
else {
qt += size;
st += (long) w.nsteals & 0xffffffffL;
if (w.isApparentlyUnblocked())
++rc;
}
}
}
}
int md = mode;
int pc = (md & SMASK);
long c = ctl;
int tc = pc + (short) (c >>> TC_SHIFT);
int ac = pc + (int) (c >> RC_SHIFT);
if (ac < 0) // ignore transient negative
ac = 0;
String level = ((md & TERMINATED) != 0 ? "Terminated"
: (md & STOP) != 0 ? "Terminating" : (md & SHUTDOWN) != 0 ? "Shutting down" : "Running");
return super.toString() + "[" + level + ", parallelism = " + pc + ", size = " + tc
+ ", active = " + ac + ", running = " + rc + ", steals = " + st + ", tasks = " + qt
+ ", submissions = " + qs + "]";
}
/**
* Possibly initiates an orderly shutdown in which previously submitted tasks are executed, but no
* new tasks will be accepted. Invocation has no effect on execution state if this is the
* {@link #commonPool()}, and no additional effect if already shut down. Tasks that are in the
* process of being submitted concurrently during the course of this method may or may not be
* rejected.
*
* @throws SecurityException if a security manager exists and the caller is not permitted to
* modify threads because it does not hold
* {@link java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public void shutdown() {
checkPermission();
tryTerminate(false, true);
}
/**
* Possibly attempts to cancel and/or stop all tasks, and reject all subsequently submitted tasks.
* Invocation has no effect on execution state if this is the {@link #commonPool()}, and no
* additional effect if already shut down. Otherwise, tasks that are in the process of being
* submitted or executed concurrently during the course of this method may or may not be rejected.
* This method cancels both existing and unexecuted tasks, in order to permit termination in the
* presence of task dependencies. So the method always returns an empty list (unlike the case for
* some other Executors).
*
* @return an empty list
* @throws SecurityException if a security manager exists and the caller is not permitted to
* modify threads because it does not hold
* {@link java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public List shutdownNow() {
checkPermission();
tryTerminate(true, true);
return Collections.emptyList();
}
/**
* Returns {@code true} if all tasks have completed following shut down.
*
* @return {@code true} if all tasks have completed following shut down
*/
public boolean isTerminated() {
return (mode & TERMINATED) != 0;
}
/**
* Returns {@code true} if the process of termination has commenced but not yet completed. This
* method may be useful for debugging. A return of {@code true} reported a sufficient period after
* shutdown may indicate that submitted tasks have ignored or suppressed interruption, or are
* waiting for I/O, causing this executor not to properly terminate. (See the advisory notes for
* class {@link ForkJoinTask} stating that tasks should not normally entail blocking operations.
* But if they do, they must abort them on interrupt.)
*
* @return {@code true} if terminating but not yet terminated
*/
public boolean isTerminating() {
int md = mode;
return (md & STOP) != 0 && (md & TERMINATED) == 0;
}
/**
* Returns {@code true} if this pool has been shut down.
*
* @return {@code true} if this pool has been shut down
*/
public boolean isShutdown() {
return (mode & SHUTDOWN) != 0;
}
// AbstractExecutorService overrides. These rely on undocumented
// fact that ForkJoinTask.adapt returns ForkJoinTasks that also
// implement RunnableFuture.
/**
* Blocks until all tasks have completed execution after a shutdown request, or the timeout
* occurs, or the current thread is interrupted, whichever happens first. Because the
* {@link #commonPool()} never terminates until program shutdown, when applied to the common pool,
* this method is equivalent to {@link #awaitQuiescence(long, TimeUnit)} but always returns
* {@code false}.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return {@code true} if this executor terminated and {@code false} if the timeout elapsed
* before termination
* @throws InterruptedException if interrupted while waiting
*/
public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (this == common) {
awaitQuiescence(timeout, unit);
return false;
}
long nanos = unit.toNanos(timeout);
if (isTerminated())
return true;
if (nanos <= 0L)
return false;
long deadline = System.nanoTime() + nanos;
synchronized (this) {
for (;;) {
if (isTerminated())
return true;
if (nanos <= 0L)
return false;
long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
wait(millis > 0L ? millis : 1L);
nanos = deadline - System.nanoTime();
}
}
}
/**
* If called by a ForkJoinTask operating in this pool, equivalent in effect to
* {@link ForkJoinTask#helpQuiesce}. Otherwise, waits and/or attempts to assist performing tasks
* until this pool {@link #isQuiescent} or the indicated timeout elapses.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return {@code true} if quiescent; {@code false} if the timeout elapsed.
*/
public boolean awaitQuiescence(long timeout, TimeUnit unit) {
long nanos = unit.toNanos(timeout);
ForkJoinWorkerThread wt;
Thread thread = Thread.currentThread();
if ((thread instanceof ForkJoinWorkerThread)
&& (wt = (ForkJoinWorkerThread) thread).pool == this) {
helpQuiescePool(wt.workQueue);
return true;
} else {
for (long startTime = System.nanoTime();;) {
ForkJoinTask t;
if ((t = pollScan(false)) != null)
t.doExec();
else if (isQuiescent())
return true;
else if ((System.nanoTime() - startTime) > nanos)
return false;
else
Thread.yield(); // cannot block
}
}
}
protected RunnableFuture newTaskFor(Runnable runnable, T value) {
return new ForkJoinTask.AdaptedRunnable(runnable, value);
}
protected RunnableFuture newTaskFor(Callable callable) {
return new ForkJoinTask.AdaptedCallable(callable);
}
/**
* Factory for creating new {@link ForkJoinWorkerThread}s. A {@code ForkJoinWorkerThreadFactory}
* must be defined and used for {@code ForkJoinWorkerThread} subclasses that extend base
* functionality or initialize threads with different contexts.
*/
public static interface ForkJoinWorkerThreadFactory {
/**
* Returns a new worker thread operating in the given pool. Returning null or throwing an
* exception may result in tasks never being executed. If this method throws an exception, it is
* relayed to the caller of the method (for example {@code execute}) causing attempted thread
* creation. If this method returns null or throws an exception, it is not retried until the
* next attempted creation (for example another call to {@code execute}).
*
* @param pool the pool this thread works in
* @return the new worker thread, or {@code null} if the request to create a thread is rejected
* @throws NullPointerException if the pool is null
*/
public ForkJoinWorkerThread newThread(ForkJoinPool pool);
}
/**
* Interface for extending managed parallelism for tasks running in {@link ForkJoinPool}s.
*
*
* A {@code ManagedBlocker} provides two methods. Method {@link #isReleasable} must return
* {@code true} if blocking is not necessary. Method {@link #block} blocks the current thread if
* necessary (perhaps internally invoking {@code isReleasable} before actually blocking). These
* actions are performed by any thread invoking {@link ForkJoinPool#managedBlock(ManagedBlocker)}.
* The unusual methods in this API accommodate synchronizers that may, but don't usually, block
* for long periods. Similarly, they allow more efficient internal handling of cases in which
* additional workers may be, but usually are not, needed to ensure sufficient parallelism. Toward
* this end, implementations of method {@code isReleasable} must be amenable to repeated
* invocation.
*
*
* For example, here is a ManagedBlocker based on a ReentrantLock:
*
*
* {
* @code
* class ManagedLocker implements ManagedBlocker {
* final ReentrantLock lock;
* boolean hasLock = false;
*
* ManagedLocker(ReentrantLock lock) {
* this.lock = lock;
* }
*
* public boolean block() {
* if (!hasLock)
* lock.lock();
* return true;
* }
*
* public boolean isReleasable() {
* return hasLock || (hasLock = lock.tryLock());
* }
* }
* }
*
*
*
* Here is a class that possibly blocks waiting for an item on a given queue:
*
*
* {
* @code
* class QueueTaker implements ManagedBlocker {
* final BlockingQueue queue;
* volatile E item = null;
*
* QueueTaker(BlockingQueue q) {
* this.queue = q;
* }
*
* public boolean block() throws InterruptedException {
* if (item == null)
* item = queue.take();
* return true;
* }
*
* public boolean isReleasable() {
* return item != null || (item = queue.poll()) != null;
* }
*
* public E getItem() { // call after pool.managedBlock completes
* return item;
* }
* }
* }
*
*/
public static interface ManagedBlocker {
/**
* Possibly blocks the current thread, for example waiting for a lock or condition.
*
* @return {@code true} if no additional blocking is necessary (i.e., if isReleasable would
* return true)
* @throws InterruptedException if interrupted while waiting (the method is not required to do
* so, but is allowed to)
*/
boolean block() throws InterruptedException;
/**
* Returns {@code true} if blocking is unnecessary.
*
* @return {@code true} if blocking is unnecessary
*/
boolean isReleasable();
}
/**
* Default ForkJoinWorkerThreadFactory implementation; creates a new ForkJoinWorkerThread using
* the system class loader as the thread context class loader.
*/
private static final class DefaultForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
private static final AccessControlContext ACC = contextWithPermissions(
// new RuntimePermission("setContextClassLoader"), // java9-concurrent-backport changed
new RuntimePermission("getClassLoader"));
public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
return AccessController.doPrivileged(new PrivilegedAction() {
public ForkJoinWorkerThread run() {
return new ForkJoinWorkerThread(pool, ClassLoader.getSystemClassLoader());
}
}, ACC);
}
}
/**
* Default ForkJoinWorkerThreadFactory implementation; creates a new ForkJoinWorkerThread using
* the system class loader as the thread context class loader.
*/
public static final class AlluxioForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
private static final AccessControlContext ACC = contextWithPermissions(
// new RuntimePermission("setContextClassLoader"), // java9-concurrent-backport changed
new RuntimePermission("getClassLoader"));
// ForkJoinWorkerThread index counter.
private static final AtomicLong sThreadIndex = new AtomicLong(0);
// Thread properties.
private final String mNameFormat;
private final boolean mIsDaemon;
/**
* Creates a new thread-factory for {@link ForkJoinPool}.
*
* @param threadNameFormat thread name format
* @param isDaemon is daemon
*/
public AlluxioForkJoinWorkerThreadFactory(String threadNameFormat, boolean isDaemon){
mNameFormat = threadNameFormat;
mIsDaemon = isDaemon;
}
public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
return AccessController.doPrivileged(new PrivilegedAction() {
public ForkJoinWorkerThread run() {
ForkJoinWorkerThread th =
new ForkJoinWorkerThread(pool, ClassLoader.getSystemClassLoader());
th.setName(String.format(mNameFormat, sThreadIndex.getAndIncrement()));
th.setDaemon(mIsDaemon);
return th;
}
}, ACC);
}
}
/**
* Queues supporting work-stealing as well as external task submission. See above for descriptions
* and algorithms. Performance on most platforms is very sensitive to placement of instances of
* both WorkQueues and their arrays -- we absolutely do not want multiple WorkQueue instances or
* multiple queue arrays sharing cache lines.
*/
// For now, using manual padding.
static final class WorkQueue {
/**
* Capacity of work-stealing queue array upon initialization. Must be a power of two; at least
* 4, but should be larger to reduce or eliminate cacheline sharing among queues. Currently, it
* is much larger, as a partial workaround for the fact that JVMs often place arrays in
* locations that share GC bookkeeping (especially cardmarks) such that per-write accesses
* encounter serious memory contention.
*/
static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
/**
* Maximum size for queue arrays. Must be a power of two less than or equal to 1 << (31 - width
* of array entry) to ensure lack of wraparound of index calculations, but defined to a value a
* bit less than this to help users trap runaway programs before saturating systems.
*/
static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M
// Unsafe mechanics. Note that some are (and must be) the same as in FJP
private static final sun.misc.Unsafe U = UnsafeAccess.unsafe;
private static final long PHASE;
private static final int ABASE;
private static final int ASHIFT;
static {
try {
PHASE = U.objectFieldOffset(WorkQueue.class.getDeclaredField("phase"));
ABASE = U.arrayBaseOffset(ForkJoinTask[].class);
int scale = U.arrayIndexScale(ForkJoinTask[].class);
if ((scale & (scale - 1)) != 0)
throw new ExceptionInInitializerError("array index scale not a power of two");
ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
} catch (Exception e) {
throw new ExceptionInInitializerError(e);
}
}
final ForkJoinPool pool; // the containing pool (may be null)
final ForkJoinWorkerThread owner; // owning thread or null if shared
// Instance fields
volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06, pad07;
volatile long pad08, pad09, pad0a, pad0b, pad0c, pad0d, pad0e, pad0f;
volatile int phase; // versioned, negative: queued, 1: locked
int stackPred; // pool stack (ctl) predecessor link
int nsteals; // number of steals
int id; // index, mode, tag
volatile int source; // source queue id, or sentinel
volatile int base; // index of next slot for poll
int top; // index of next slot for push
ForkJoinTask[] array; // the elements (initially unallocated)
volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17;
volatile Object pad18, pad19, pad1a, pad1b, pad1c, pad1d, pad1e, pad1f;
WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner) {
this.pool = pool;
this.owner = owner;
// Place indices in the center of array (that is not yet allocated)
base = top = INITIAL_QUEUE_CAPACITY >>> 1;
}
/**
* Returns an exportable index (used by ForkJoinWorkerThread).
*/
final int getPoolIndex() {
return (id & 0xffff) >>> 1; // ignore odd/even tag bit
}
/**
* Returns the approximate number of tasks in the queue.
*/
final int queueSize() {
int n = base - top; // read base first
return (n >= 0) ? 0 : -n; // ignore transient negative
}
/**
* Provides a more accurate estimate of whether this queue has any tasks than does queueSize, by
* checking whether a near-empty queue has at least one unclaimed task.
*/
final boolean isEmpty() {
ForkJoinTask[] a;
int n, al, b;
return ((n = (b = base) - top) >= 0 || // possibly one task
(n == -1 && ((a = array) == null || (al = a.length) == 0 || a[(al - 1) & b] == null)));
}
/**
* Pushes a task. Call only by owner in unshared queues.
*
* @param task the task. Caller must ensure non-null.
* @throws RejectedExecutionException if array cannot be resized
*/
final void push(ForkJoinTask task) {
int s = top;
ForkJoinTask[] a;
int al, d;
if ((a = array) != null && (al = a.length) > 0) {
int index = (al - 1) & s;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinPool p = pool;
top = s + 1;
U.putOrderedObject(a, offset, task);
if ((d = base - s) == 0 && p != null) {
U.fullFence();
p.signalWork();
} else if (d + al == 1)
growArray();
}
}
/**
* Initializes or doubles the capacity of array. Call either by owner or with lock held -- it is
* OK for base, but not top, to move while resizings are in progress.
*/
final ForkJoinTask[] growArray() {
ForkJoinTask[] oldA = array;
int oldSize = oldA != null ? oldA.length : 0;
int size = oldSize > 0 ? oldSize << 1 : INITIAL_QUEUE_CAPACITY;
if (size < INITIAL_QUEUE_CAPACITY || size > MAXIMUM_QUEUE_CAPACITY)
throw new RejectedExecutionException("Queue capacity exceeded");
int oldMask, t, b;
ForkJoinTask[] a = array = new ForkJoinTask[size];
if (oldA != null && (oldMask = oldSize - 1) > 0 && (t = top) - (b = base) > 0) {
int mask = size - 1;
do { // emulate poll from old array, push to new array
int index = b & oldMask;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask x = (ForkJoinTask) U.getObjectVolatile(oldA, offset);
if (x != null && U.compareAndSwapObject(oldA, offset, x, null))
a[b & mask] = x;
} while (++b != t);
U.storeFence();
}
return a;
}
/**
* Takes next task, if one exists, in LIFO order. Call only by owner in unshared queues.
*/
final ForkJoinTask pop() {
int b = base, s = top, al;
ForkJoinTask[] a;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & --s;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getObject(a, offset);
if (t != null && U.compareAndSwapObject(a, offset, t, null)) {
top = s;
U.storeFence();
return t;
}
}
return null;
}
// Specialized execution methods
/**
* Takes next task, if one exists, in FIFO order.
*/
final ForkJoinTask poll() {
for (;;) {
int b = base, s = top, d, al;
ForkJoinTask[] a;
if ((a = array) != null && (d = b - s) < 0 && (al = a.length) > 0) {
int index = (al - 1) & b;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getObjectVolatile(a, offset);
if (b++ == base) {
if (t != null) {
if (U.compareAndSwapObject(a, offset, t, null)) {
base = b;
return t;
}
} else if (d == -1)
break; // now empty
}
} else
break;
}
return null;
}
/**
* Takes next task, if one exists, in order specified by mode.
*/
final ForkJoinTask nextLocalTask() {
return ((id & FIFO) != 0) ? poll() : pop();
}
/**
* Returns next task, if one exists, in order specified by mode.
*/
final ForkJoinTask peek() {
int al;
ForkJoinTask[] a;
return ((a = array) != null && (al = a.length) > 0)
? a[(al - 1) & ((id & FIFO) != 0 ? base : top - 1)]
: null;
}
/**
* Pops the given task only if it is at the current top.
*/
final boolean tryUnpush(ForkJoinTask task) {
int b = base, s = top, al;
ForkJoinTask[] a;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & --s;
long offset = ((long) index << ASHIFT) + ABASE;
if (U.compareAndSwapObject(a, offset, task, null)) {
top = s;
U.storeFence();
return true;
}
}
return false;
}
// Operations on shared queues
/**
* Removes and cancels all known tasks, ignoring any exceptions.
*/
final void cancelAll() {
for (ForkJoinTask t; (t = poll()) != null;)
ForkJoinTask.cancelIgnoringExceptions(t);
}
/**
* Pops and executes up to limit consecutive tasks or until empty.
*
* @param limit max runs, or zero for no limit
*/
final void localPopAndExec(int limit) {
for (;;) {
int b = base, s = top, al;
ForkJoinTask[] a;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & --s;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getAndSetObject(a, offset, null);
if (t != null) {
top = s;
U.storeFence();
t.doExec();
if (limit != 0 && --limit == 0)
break;
} else
break;
} else
break;
}
}
/**
* Polls and executes up to limit consecutive tasks or until empty.
*
* @param limit, or zero for no limit
*/
final void localPollAndExec(int limit) {
for (int polls = 0;;) {
int b = base, s = top, d, al;
ForkJoinTask[] a;
if ((a = array) != null && (d = b - s) < 0 && (al = a.length) > 0) {
int index = (al - 1) & b++;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getAndSetObject(a, offset, null);
if (t != null) {
base = b;
t.doExec();
if (limit != 0 && ++polls == limit)
break;
} else if (d == -1)
break; // now empty
else
polls = 0; // stolen; reset
} else
break;
}
}
/**
* If present, removes task from queue and executes it.
*/
final void tryRemoveAndExec(ForkJoinTask task) {
ForkJoinTask[] wa;
int s, wal;
if (base - (s = top) < 0 && // traverse from top
(wa = array) != null && (wal = wa.length) > 0) {
for (int m = wal - 1, ns = s - 1, i = ns;; --i) {
int index = i & m;
long offset = (index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getObject(wa, offset);
if (t == null)
break;
else if (t == task) {
if (U.compareAndSwapObject(wa, offset, t, null)) {
top = ns; // safely shift down
for (int j = i; j != ns; ++j) {
ForkJoinTask f;
int pindex = (j + 1) & m;
long pOffset = (pindex << ASHIFT) + ABASE;
f = (ForkJoinTask) U.getObject(wa, pOffset);
U.putObjectVolatile(wa, pOffset, null);
int jindex = j & m;
long jOffset = (jindex << ASHIFT) + ABASE;
U.putOrderedObject(wa, jOffset, f);
}
U.storeFence();
t.doExec();
}
break;
}
}
}
}
/**
* Tries to steal and run tasks within the target's computation until done, not found, or limit
* exceeded.
*
* @param task root of CountedCompleter computation
* @param limit max runs, or zero for no limit
* @return task status on exit
*/
final int localHelpCC(CountedCompleter task, int limit) {
int status = 0;
if (task != null && (status = task.status) >= 0) {
for (;;) {
boolean help = false;
int b = base, s = top, al;
ForkJoinTask[] a;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & (s - 1);
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask o = (ForkJoinTask) U.getObject(a, offset);
if (o instanceof CountedCompleter) {
CountedCompleter t = (CountedCompleter) o;
for (CountedCompleter f = t;;) {
if (f != task) {
if ((f = f.completer) == null) // try parent
break;
} else {
if (U.compareAndSwapObject(a, offset, t, null)) {
top = s - 1;
U.storeFence();
t.doExec();
help = true;
}
break;
}
}
}
}
if ((status = task.status) < 0 || !help || (limit != 0 && --limit == 0))
break;
}
}
return status;
}
/**
* Tries to lock shared queue by CASing phase field.
*/
final boolean tryLockSharedQueue() {
return U.compareAndSwapInt(this, PHASE, 0, QLOCK);
}
/**
* Shared version of tryUnpush.
*/
final boolean trySharedUnpush(ForkJoinTask task) {
boolean popped = false;
int s = top - 1, al;
ForkJoinTask[] a;
if ((a = array) != null && (al = a.length) > 0) {
int index = (al - 1) & s;
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask t = (ForkJoinTask) U.getObject(a, offset);
if (t == task && U.compareAndSwapInt(this, PHASE, 0, QLOCK)) {
if (top == s + 1 && array == a && U.compareAndSwapObject(a, offset, task, null)) {
popped = true;
top = s;
}
U.putOrderedInt(this, PHASE, 0);
}
}
return popped;
}
/**
* Shared version of localHelpCC.
*/
final int sharedHelpCC(CountedCompleter task, int limit) {
int status = 0;
if (task != null && (status = task.status) >= 0) {
for (;;) {
boolean help = false;
int b = base, s = top, al;
ForkJoinTask[] a;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & (s - 1);
long offset = ((long) index << ASHIFT) + ABASE;
ForkJoinTask o = (ForkJoinTask) U.getObject(a, offset);
if (o instanceof CountedCompleter) {
CountedCompleter t = (CountedCompleter) o;
for (CountedCompleter f = t;;) {
if (f != task) {
if ((f = f.completer) == null)
break;
} else {
if (U.compareAndSwapInt(this, PHASE, 0, QLOCK)) {
if (top == s && array == a && U.compareAndSwapObject(a, offset, t, null)) {
help = true;
top = s - 1;
}
U.putOrderedInt(this, PHASE, 0);
if (help)
t.doExec();
}
break;
}
}
}
}
if ((status = task.status) < 0 || !help || (limit != 0 && --limit == 0))
break;
}
}
return status;
}
/**
* Returns true if owned and not known to be blocked.
*/
final boolean isApparentlyUnblocked() {
Thread wt;
Thread.State s;
return ((wt = owner) != null && (s = wt.getState()) != Thread.State.BLOCKED
&& s != Thread.State.WAITING && s != Thread.State.TIMED_WAITING);
}
}
/**
* Factory for innocuous worker threads.
*/
private static final class InnocuousForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
/**
* An ACC to restrict permissions for the factory itself. The constructed workers have no
* permissions set.
*/
private static final AccessControlContext ACC = contextWithPermissions(modifyThreadPermission,
new RuntimePermission("enableContextClassLoaderOverride"),
new RuntimePermission("modifyThreadGroup"), new RuntimePermission("getClassLoader"),
new RuntimePermission("setContextClassLoader"));
public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
return AccessController.doPrivileged(new PrivilegedAction() {
public ForkJoinWorkerThread run() {
return new ForkJoinWorkerThread.InnocuousForkJoinWorkerThread(pool);
}
}, ACC);
}
}
}