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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/
*/
/*
* Source:
* http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/Striped64.java?revision=1.9
*/
package com.google.common.cache;
import java.util.Random;
/**
* A package-local class holding common representation and mechanics
* for classes supporting dynamic striping on 64bit values. The class
* extends Number so that concrete subclasses must publicly do so.
*/
abstract class Striped64 extends Number {
/*
* This class maintains a lazily-initialized table of atomically
* updated variables, plus an extra "base" field. The table size
* is a power of two. Indexing uses masked per-thread hash codes.
* Nearly all declarations in this class are package-private,
* accessed directly by subclasses.
*
* Table entries are of class Cell; a variant of AtomicLong padded
* to reduce cache contention on most processors. Padding is
* overkill for most Atomics because they are usually irregularly
* scattered in memory and thus don't interfere much with each
* other. But Atomic objects residing in arrays will tend to be
* placed adjacent to each other, and so will most often share
* cache lines (with a huge negative performance impact) without
* this precaution.
*
* In part because Cells are relatively large, we avoid creating
* them until they are needed. When there is no contention, all
* updates are made to the base field. Upon first contention (a
* failed CAS on base update), the table is initialized to size 2.
* The table size is doubled upon further contention until
* reaching the nearest power of two greater than or equal to the
* number of CPUS. Table slots remain empty (null) until they are
* needed.
*
* A single spinlock ("busy") is used for initializing and
* resizing the table, as well as populating slots with new Cells.
* There is no need for a blocking lock; when the lock is not
* available, threads try other slots (or the base). During these
* retries, there is increased contention and reduced locality,
* which is still better than alternatives.
*
* Per-thread hash codes are initialized to random values.
* Contention and/or table collisions are indicated by failed
* CASes when performing an update operation (see method
* retryUpdate). Upon a collision, if the table size is less than
* the capacity, it is doubled in size unless some other thread
* holds the lock. If a hashed slot is empty, and lock is
* available, a new Cell is created. Otherwise, if the slot
* exists, a CAS is tried. Retries proceed by "double hashing",
* using a secondary hash (Marsaglia XorShift) to try to find a
* free slot.
*
* The table size is capped because, when there are more threads
* than CPUs, supposing that each thread were bound to a CPU,
* there would exist a perfect hash function mapping threads to
* slots that eliminates collisions. When we reach capacity, we
* search for this mapping by randomly varying the hash codes of
* colliding threads. Because search is random, and collisions
* only become known via CAS failures, convergence can be slow,
* and because threads are typically not bound to CPUS forever,
* may not occur at all. However, despite these limitations,
* observed contention rates are typically low in these cases.
*
* It is possible for a Cell to become unused when threads that
* once hashed to it terminate, as well as in the case where
* doubling the table causes no thread to hash to it under
* expanded mask. We do not try to detect or remove such cells,
* under the assumption that for long-running instances, observed
* contention levels will recur, so the cells will eventually be
* needed again; and for short-lived ones, it does not matter.
*/
/**
* Padded variant of AtomicLong supporting only raw accesses plus CAS.
* The value field is placed between pads, hoping that the JVM doesn't
* reorder them.
*
* JVM intrinsics note: It would be possible to use a release-only
* form of CAS here, if it were provided.
*/
static final class Cell {
volatile long p0, p1, p2, p3, p4, p5, p6;
volatile long value;
volatile long q0, q1, q2, q3, q4, q5, q6;
Cell(long x) { value = x; }
final boolean cas(long cmp, long val) {
return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long valueOffset;
static {
try {
UNSAFE = getUnsafe();
Class ak = Cell.class;
valueOffset = UNSAFE.objectFieldOffset
(ak.getDeclaredField("value"));
} catch (Exception e) {
throw new Error(e);
}
}
}
/**
* ThreadLocal holding a single-slot int array holding hash code.
* Unlike the JDK8 version of this class, we use a suboptimal
* int[] representation to avoid introducing a new type that can
* impede class-unloading when ThreadLocals are not removed.
*/
static final ThreadLocal threadHashCode = new ThreadLocal();
/**
* Generator of new random hash codes
*/
static final Random rng = new Random();
/** Number of CPUS, to place bound on table size */
static final int NCPU = Runtime.getRuntime().availableProcessors();
/**
* Table of cells. When non-null, size is a power of 2.
*/
transient volatile Cell[] cells;
/**
* Base value, used mainly when there is no contention, but also as
* a fallback during table initialization races. Updated via CAS.
*/
transient volatile long base;
/**
* Spinlock (locked via CAS) used when resizing and/or creating Cells.
*/
transient volatile int busy;
/**
* Package-private default constructor
*/
Striped64() {
}
/**
* CASes the base field.
*/
final boolean casBase(long cmp, long val) {
return UNSAFE.compareAndSwapLong(this, baseOffset, cmp, val);
}
/**
* CASes the busy field from 0 to 1 to acquire lock.
*/
final boolean casBusy() {
return UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1);
}
/**
* Computes the function of current and new value. Subclasses
* should open-code this update function for most uses, but the
* virtualized form is needed within retryUpdate.
*
* @param currentValue the current value (of either base or a cell)
* @param newValue the argument from a user update call
* @return result of the update function
*/
abstract long fn(long currentValue, long newValue);
/**
* Handles cases of updates involving initialization, resizing,
* creating new Cells, and/or contention. See above for
* explanation. This method suffers the usual non-modularity
* problems of optimistic retry code, relying on rechecked sets of
* reads.
*
* @param x the value
* @param hc the hash code holder
* @param wasUncontended false if CAS failed before call
*/
final void retryUpdate(long x, int[] hc, boolean wasUncontended) {
int h;
if (hc == null) {
threadHashCode.set(hc = new int[1]); // Initialize randomly
int r = rng.nextInt(); // Avoid zero to allow xorShift rehash
h = hc[0] = (r == 0) ? 1 : r;
}
else
h = hc[0];
boolean collide = false; // True if last slot nonempty
for (;;) {
Cell[] as; Cell a; int n; long v;
if ((as = cells) != null && (n = as.length) > 0) {
if ((a = as[(n - 1) & h]) == null) {
if (busy == 0) { // Try to attach new Cell
Cell r = new Cell(x); // Optimistically create
if (busy == 0 && casBusy()) {
boolean created = false;
try { // Recheck under lock
Cell[] rs; int m, j;
if ((rs = cells) != null &&
(m = rs.length) > 0 &&
rs[j = (m - 1) & h] == null) {
rs[j] = r;
created = true;
}
} finally {
busy = 0;
}
if (created)
break;
continue; // Slot is now non-empty
}
}
collide = false;
}
else if (!wasUncontended) // CAS already known to fail
wasUncontended = true; // Continue after rehash
else if (a.cas(v = a.value, fn(v, x)))
break;
else if (n >= NCPU || cells != as)
collide = false; // At max size or stale
else if (!collide)
collide = true;
else if (busy == 0 && casBusy()) {
try {
if (cells == as) { // Expand table unless stale
Cell[] rs = new Cell[n << 1];
for (int i = 0; i < n; ++i)
rs[i] = as[i];
cells = rs;
}
} finally {
busy = 0;
}
collide = false;
continue; // Retry with expanded table
}
h ^= h << 13; // Rehash
h ^= h >>> 17;
h ^= h << 5;
hc[0] = h; // Record index for next time
}
else if (busy == 0 && cells == as && casBusy()) {
boolean init = false;
try { // Initialize table
if (cells == as) {
Cell[] rs = new Cell[2];
rs[h & 1] = new Cell(x);
cells = rs;
init = true;
}
} finally {
busy = 0;
}
if (init)
break;
}
else if (casBase(v = base, fn(v, x)))
break; // Fall back on using base
}
}
/**
* Sets base and all cells to the given value.
*/
final void internalReset(long initialValue) {
Cell[] as = cells;
base = initialValue;
if (as != null) {
int n = as.length;
for (int i = 0; i < n; ++i) {
Cell a = as[i];
if (a != null)
a.value = initialValue;
}
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long baseOffset;
private static final long busyOffset;
static {
try {
UNSAFE = getUnsafe();
Class sk = Striped64.class;
baseOffset = UNSAFE.objectFieldOffset
(sk.getDeclaredField("base"));
busyOffset = UNSAFE.objectFieldOffset
(sk.getDeclaredField("busy"));
} catch (Exception e) {
throw new Error(e);
}
}
/**
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
* Replace with a simple call to Unsafe.getUnsafe when integrating
* into a jdk.
*
* @return a sun.misc.Unsafe
*/
private static sun.misc.Unsafe getUnsafe() {
try {
return sun.misc.Unsafe.getUnsafe();
} catch (SecurityException tryReflectionInstead) {}
try {
return java.security.AccessController.doPrivileged
(new java.security.PrivilegedExceptionAction() {
public sun.misc.Unsafe run() throws Exception {
Class k = sun.misc.Unsafe.class;
for (java.lang.reflect.Field f : k.getDeclaredFields()) {
f.setAccessible(true);
Object x = f.get(null);
if (k.isInstance(x))
return k.cast(x);
}
throw new NoSuchFieldError("the Unsafe");
}});
} catch (java.security.PrivilegedActionException e) {
throw new RuntimeException("Could not initialize intrinsics",
e.getCause());
}
}
}