org.jctools.maps.NonBlockingIdentityHashMap Maven / Gradle / Ivy
/*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.jctools.maps;
import org.jctools.util.RangeUtil;
import java.io.IOException;
import java.io.Serializable;
import java.util.*;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicLongFieldUpdater;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
import static org.jctools.maps.NonBlockingHashMap.DUMMY_VOLATILE;
import static org.jctools.util.UnsafeAccess.UNSAFE;
import static org.jctools.util.UnsafeAccess.fieldOffset;
/**
* A lock-free alternate implementation of {@link java.util.concurrent.ConcurrentHashMap}
* with better scaling properties and generally lower costs to mutate the Map.
* It provides identical correctness properties as ConcurrentHashMap. All
* operations are non-blocking and multi-thread safe, including all update
* operations. {@link NonBlockingHashMap} scales substatially better than
* {@link java.util.concurrent.ConcurrentHashMap} for high update rates, even with a
* large concurrency factor. Scaling is linear up to 768 CPUs on a 768-CPU
* Azul box, even with 100% updates or 100% reads or any fraction in-between.
* Linear scaling up to all cpus has been observed on a 32-way Sun US2 box,
* 32-way Sun Niagra box, 8-way Intel box and a 4-way Power box.
*
* This class obeys the same functional specification as {@link
* java.util.Hashtable}, and includes versions of methods corresponding to
* each method of Hashtable. However, even though all operations are
* thread-safe, operations do not entail locking and there is
* not any support for locking the entire table in a way that
* prevents all access. This class is fully interoperable with
* Hashtable in programs that rely on its thread safety but not on
* its synchronization details.
*
* Operations (including put) generally do not block, so may
* overlap with other update operations (including other puts and
* removes). Retrievals reflect the results of the most recently
* completed update operations holding upon their onset. For
* aggregate operations such as putAll, concurrent retrievals may
* reflect insertion or removal of only some entries. Similarly, Iterators
* and Enumerations return elements reflecting the state of the hash table at
* some point at or since the creation of the iterator/enumeration. They do
* not throw {@link ConcurrentModificationException}. However,
* iterators are designed to be used by only one thread at a time.
*
*
Very full tables, or tables with high reprobe rates may trigger an
* internal resize operation to move into a larger table. Resizing is not
* terribly expensive, but it is not free either; during resize operations
* table throughput may drop somewhat. All threads that visit the table
* during a resize will 'help' the resizing but will still be allowed to
* complete their operation before the resize is finished (i.e., a simple
* 'get' operation on a million-entry table undergoing resizing will not need
* to block until the entire million entries are copied).
*
*
This class and its views and iterators implement all of the
* optional methods of the {@link Map} and {@link Iterator}
* interfaces.
*
*
Like {@link Hashtable} but unlike {@link HashMap}, this class
* does not allow null to be used as a key or value.
*
*
* @since 1.5
* @author Cliff Click
* @param the type of keys maintained by this map
* @param the type of mapped values
*
* @author Prashant Deva
* Modified from original NonBlockingHashMap to use identity equality.
* Uses System.identityHashCode() to calculate hashMap.
* Key equality is compared using '=='.
*/
public class NonBlockingIdentityHashMap
extends AbstractMap
implements ConcurrentMap, Cloneable, Serializable {
private static final long serialVersionUID = 1234123412341234123L;
private static final int REPROBE_LIMIT=10; // Too many reprobes then force a table-resize
// --- Bits to allow Unsafe access to arrays
private static final int _Obase = UNSAFE.arrayBaseOffset(Object[].class);
private static final int _Oscale = UNSAFE.arrayIndexScale(Object[].class);
private static long rawIndex(final Object[] ary, final int idx) {
assert idx >= 0 && idx < ary.length;
return _Obase + (idx * (long)_Oscale);
}
// --- Setup to use Unsafe
private static final long _kvs_offset = fieldOffset(NonBlockingHashMap.class, "_kvs");
private final boolean CAS_kvs( final Object[] oldkvs, final Object[] newkvs ) {
return UNSAFE.compareAndSwapObject(this, _kvs_offset, oldkvs, newkvs );
}
// --- Adding a 'prime' bit onto Values via wrapping with a junk wrapper class
private static final class Prime {
final Object _V;
Prime( Object V ) { _V = V; }
static Object unbox( Object V ) { return V instanceof Prime ? ((Prime)V)._V : V; }
}
// --- hash ----------------------------------------------------------------
// Helper function to spread lousy hashCodes
private static final int hash(final Object key) {
if (key == null) throw new NullPointerException();
int h = System.identityHashCode(key); // The real hashCode call
// I assume that System.identityHashCode is well implemented with a good
// spreader, and a second bit-spreader is redundant.
return h;
}
// --- The Hash Table --------------------
// Slot 0 is always used for a 'CHM' entry below to hold the interesting
// bits of the hash table. Slot 1 holds full hashes as an array of ints.
// Slots {2,3}, {4,5}, etc hold {Key,Value} pairs. The entire hash table
// can be atomically replaced by CASing the _kvs field.
//
// Why is CHM buried inside the _kvs Object array, instead of the other way
// around? The CHM info is used during resize events and updates, but not
// during standard 'get' operations. I assume 'get' is much more frequent
// than 'put'. 'get' can skip the extra indirection of skipping through the
// CHM to reach the _kvs array.
private transient Object[] _kvs;
private static final CHM chm (Object[] kvs) { return (CHM )kvs[0]; }
private static final int[] hashes(Object[] kvs) { return (int[])kvs[1]; }
// Number of K,V pairs in the table
private static final int len(Object[] kvs) { return (kvs.length-2)>>1; }
// Time since last resize
private transient long _last_resize_milli;
// --- Minimum table size ----------------
// Pick size 8 K/V pairs, which turns into (8*2+2)*4+12 = 84 bytes on a
// standard 32-bit HotSpot, and (8*2+2)*8+12 = 156 bytes on 64-bit Azul.
private static final int MIN_SIZE_LOG=3; //
private static final int MIN_SIZE=(1<>2);
}
// --- NonBlockingHashMap --------------------------------------------------
// Constructors
/** Create a new NonBlockingHashMap with default minimum size (currently set
* to 8 K/V pairs or roughly 84 bytes on a standard 32-bit JVM). */
public NonBlockingIdentityHashMap( ) { this(MIN_SIZE); }
/** Create a new NonBlockingHashMap with initial room for the given number of
* elements, thus avoiding internal resizing operations to reach an
* appropriate size. Large numbers here when used with a small count of
* elements will sacrifice space for a small amount of time gained. The
* initial size will be rounded up internally to the next larger power of 2. */
public NonBlockingIdentityHashMap( final int initial_sz ) { initialize(initial_sz); }
private final void initialize( int initial_sz ) {
RangeUtil.checkPositiveOrZero(initial_sz, "initial_sz");
int i; // Convert to next largest power-of-2
if( initial_sz > 1024*1024 ) initial_sz = 1024*1024;
for( i=MIN_SIZE_LOG; (1<size() == 0.
* @return size() == 0 */
@Override
public boolean isEmpty ( ) { return size() == 0; }
/** Tests if the key in the table using the equals method.
* @return true if the key is in the table using the equals method
* @throws NullPointerException if the specified key is null */
@Override
public boolean containsKey( Object key ) { return get(key) != null; }
/** Legacy method testing if some key maps into the specified value in this
* table. This method is identical in functionality to {@link
* #containsValue}, and exists solely to ensure full compatibility with
* class {@link java.util.Hashtable}, which supported this method prior to
* introduction of the Java Collections framework.
* @param val a value to search for
* @return true if this map maps one or more keys to the specified value
* @throws NullPointerException if the specified value is null */
public boolean contains ( Object val ) { return containsValue(val); }
/** Maps the specified key to the specified value in the table. Neither key
* nor value can be null.
* The value can be retrieved by calling {@link #get} with a key that is
* equal to the original key.
* @param key key with which the specified value is to be associated
* @param val value to be associated with the specified key
* @return the previous value associated with key, or
* null if there was no mapping for key
* @throws NullPointerException if the specified key or value is null */
@Override
public TypeV put ( TypeK key, TypeV val ) { return putIfMatch( key, val, NO_MATCH_OLD); }
/** Atomically, do a {@link #put} if-and-only-if the key is not mapped.
* Useful to ensure that only a single mapping for the key exists, even if
* many threads are trying to create the mapping in parallel.
* @return the previous value associated with the specified key,
* or null if there was no mapping for the key
* @throws NullPointerException if the specified key or value is null */
@Override
public TypeV putIfAbsent( TypeK key, TypeV val ) { return putIfMatch( key, val, TOMBSTONE ); }
/** Removes the key (and its corresponding value) from this map.
* This method does nothing if the key is not in the map.
* @return the previous value associated with key, or
* null if there was no mapping for key
* @throws NullPointerException if the specified key is null */
@Override
public TypeV remove ( Object key ) { return putIfMatch( key,TOMBSTONE, NO_MATCH_OLD); }
/** Atomically do a {@link #remove(Object)} if-and-only-if the key is mapped
* to a value which is equals to the given value.
* @throws NullPointerException if the specified key or value is null */
public boolean remove ( Object key,Object val ) {
return objectsEquals(putIfMatch( key,TOMBSTONE, val ), val);
}
/** Atomically do a put(key,val) if-and-only-if the key is
* mapped to some value already.
* @throws NullPointerException if the specified key or value is null */
@Override
public TypeV replace ( TypeK key, TypeV val ) { return putIfMatch( key, val,MATCH_ANY ); }
/** Atomically do a put(key,newValue) if-and-only-if the key is
* mapped a value which is equals to oldValue.
* @throws NullPointerException if the specified key or value is null */
@Override
public boolean replace ( TypeK key, TypeV oldValue, TypeV newValue ) {
return objectsEquals(putIfMatch( key, newValue, oldValue ), oldValue);
}
private static boolean objectsEquals(Object a, Object b) {
return (a == b) || (a != null && a.equals(b));
}
private final TypeV putIfMatch( Object key, Object newVal, Object oldVal ) {
if (oldVal == null || newVal == null) throw new NullPointerException();
final Object res = putIfMatch0( this, _kvs, key, newVal, oldVal );
assert !(res instanceof Prime);
assert res != null;
return res == TOMBSTONE ? null : (TypeV)res;
}
/** Copies all of the mappings from the specified map to this one, replacing
* any existing mappings.
* @param m mappings to be stored in this map */
@Override
public void putAll(Map m) {
for (Map.Entry e : m.entrySet())
put(e.getKey(), e.getValue());
}
/** Removes all of the mappings from this map. */
@Override
public void clear() { // Smack a new empty table down
Object[] newkvs = new NonBlockingIdentityHashMap(MIN_SIZE)._kvs;
while( !CAS_kvs(_kvs,newkvs) ) // Spin until the clear works
;
}
/** Returns true if this Map maps one or more keys to the specified
* value. Note: This method requires a full internal traversal of the
* hash table and is much slower than {@link #containsKey}.
* @param val value whose presence in this map is to be tested
* @return true if this map maps one or more keys to the specified value
* @throws NullPointerException if the specified value is null */
@Override
public boolean containsValue( final Object val ) {
if( val == null ) throw new NullPointerException();
for( TypeV V : values() )
if( V == val || V.equals(val) )
return true;
return false;
}
// This function is supposed to do something for Hashtable, and the JCK
// tests hang until it gets called... by somebody ... for some reason,
// any reason....
protected void rehash() {
}
/**
* Creates a shallow copy of this hashtable. All the structure of the
* hashtable itself is copied, but the keys and values are not cloned.
* This is a relatively expensive operation.
*
* @return a clone of the hashtable.
*/
@Override
public Object clone() {
try {
// Must clone, to get the class right; NBHM might have been
// extended so it would be wrong to just make a new NBHM.
NonBlockingIdentityHashMap t = (NonBlockingIdentityHashMap) super.clone();
// But I don't have an atomic clone operation - the underlying _kvs
// structure is undergoing rapid change. If I just clone the _kvs
// field, the CHM in _kvs[0] won't be in sync.
//
// Wipe out the cloned array (it was shallow anyways).
t.clear();
// Now copy sanely
for( TypeK K : keySet() ) {
final TypeV V = get(K); // Do an official 'get'
t.put(K,V);
}
return t;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
}
/**
* Returns a string representation of this map. The string representation
* consists of a list of key-value mappings in the order returned by the
* map's entrySet view's iterator, enclosed in braces
* ("{}"). Adjacent mappings are separated by the characters
* ", " (comma and space). Each key-value mapping is rendered as
* the key followed by an equals sign ("=") followed by the
* associated value. Keys and values are converted to strings as by
* {@link String#valueOf(Object)}.
*
* @return a string representation of this map
*/
@Override
public String toString() {
Iterator> i = entrySet().iterator();
if( !i.hasNext())
return "{}";
StringBuilder sb = new StringBuilder();
sb.append('{');
for (;;) {
Entry e = i.next();
TypeK key = e.getKey();
TypeV value = e.getValue();
sb.append(key == this ? "(this Map)" : key);
sb.append('=');
sb.append(value == this ? "(this Map)" : value);
if( !i.hasNext())
return sb.append('}').toString();
sb.append(", ");
}
}
// --- get -----------------------------------------------------------------
/** Returns the value to which the specified key is mapped, or {@code null}
* if this map contains no mapping for the key.
* More formally, if this map contains a mapping from a key {@code k} to
* a value {@code v} such that {@code key.equals(k)}, then this method
* returns {@code v}; otherwise it returns {@code null}. (There can be at
* most one such mapping.)
* @throws NullPointerException if the specified key is null */
// Never returns a Prime nor a Tombstone.
@Override
public TypeV get( Object key ) {
final Object V = get_impl(this,_kvs,key);
assert !(V instanceof Prime); // Never return a Prime
assert V != TOMBSTONE;
return (TypeV)V;
}
private static final Object get_impl( final NonBlockingIdentityHashMap topmap, final Object[] kvs, final Object key ) {
final int fullhash= hash (key); // throws NullPointerException if key is null
final int len = len (kvs); // Count of key/value pairs, reads kvs.length
final CHM chm = chm (kvs); // The CHM, for a volatile read below; reads slot 0 of kvs
int idx = fullhash & (len-1); // First key hash
// Main spin/reprobe loop, looking for a Key hit
int reprobe_cnt=0;
while( true ) {
// Probe table. Each read of 'val' probably misses in cache in a big
// table; hopefully the read of 'key' then hits in cache.
final Object K = key(kvs,idx); // Get key before volatile read, could be null
final Object V = val(kvs,idx); // Get value before volatile read, could be null or Tombstone or Prime
if( K == null ) return null; // A clear miss
// We need a volatile-read here to preserve happens-before semantics on
// newly inserted Keys. If the Key body was written just before inserting
// into the table a Key-compare here might read the uninitialized Key body.
// Annoyingly this means we have to volatile-read before EACH key compare.
// .
// We also need a volatile-read between reading a newly inserted Value
// and returning the Value (so the user might end up reading the stale
// Value contents). Same problem as with keys - and the one volatile
// read covers both.
final Object[] newkvs = chm._newkvs; // VOLATILE READ before key compare
// Key-compare
if( K == key ) {
// Key hit! Check for no table-copy-in-progress
if( !(V instanceof Prime) ) // No copy?
return (V == TOMBSTONE) ? null : V; // Return the value
// Key hit - but slot is (possibly partially) copied to the new table.
// Finish the copy & retry in the new table.
return get_impl(topmap,chm.copy_slot_and_check(topmap,kvs,idx,key),key); // Retry in the new table
}
// get and put must have the same key lookup logic! But only 'put'
// needs to force a table-resize for a too-long key-reprobe sequence.
// Check for too-many-reprobes on get - and flip to the new table.
if( ++reprobe_cnt >= reprobe_limit(len) || // too many probes
K == TOMBSTONE ) // found a TOMBSTONE key, means no more keys in this table
return newkvs == null ? null : get_impl(topmap,topmap.help_copy(newkvs),key); // Retry in the new table
idx = (idx+1)&(len-1); // Reprobe by 1! (could now prefetch)
}
}
// --- putIfMatch ---------------------------------------------------------
// Put, Remove, PutIfAbsent, etc. Return the old value. If the returned
// value is equal to expVal (or expVal is NO_MATCH_OLD) then the put can be
// assumed to work (although might have been immediately overwritten). Only
// the path through copy_slot passes in an expected value of null, and
// putIfMatch only returns a null if passed in an expected null.
private static final Object putIfMatch0(final NonBlockingIdentityHashMap topmap, final Object[] kvs, final Object key, final Object putval, final Object expVal ) {
assert putval != null;
assert !(putval instanceof Prime);
assert !(expVal instanceof Prime);
final int fullhash = hash (key); // throws NullPointerException if key null
final int len = len (kvs); // Count of key/value pairs, reads kvs.length
final CHM chm = chm (kvs); // Reads kvs[0]
int idx = fullhash & (len-1);
// ---
// Key-Claim stanza: spin till we can claim a Key (or force a resizing).
int reprobe_cnt=0;
Object K=null, V=null;
Object[] newkvs=null;
while( true ) { // Spin till we get a Key slot
V = val(kvs,idx); // Get old value (before volatile read below!)
K = key(kvs,idx); // Get current key
if( K == null ) { // Slot is free?
// Found an empty Key slot - which means this Key has never been in
// this table. No need to put a Tombstone - the Key is not here!
if( putval == TOMBSTONE ) return TOMBSTONE; // Not-now & never-been in this table
if( expVal == MATCH_ANY ) return TOMBSTONE; // Will not match, even after K inserts
// Claim the null key-slot
if( CAS_key(kvs,idx, null, key ) ) { // Claim slot for Key
chm._slots.add(1); // Raise key-slots-used count
break; // Got it!
}
// CAS to claim the key-slot failed.
//
// This re-read of the Key points out an annoying short-coming of Java
// CAS. Most hardware CAS's report back the existing value - so that
// if you fail you have a *witness* - the value which caused the CAS to
// fail. The Java API turns this into a boolean destroying the
// witness. Re-reading does not recover the witness because another
// thread can write over the memory after the CAS. Hence we can be in
// the unfortunate situation of having a CAS fail *for cause* but
// having that cause removed by a later store. This turns a
// non-spurious-failure CAS (such as Azul has) into one that can
// apparently spuriously fail - and we avoid apparent spurious failure
// by not allowing Keys to ever change.
// Volatile read, to force loads of K to retry despite JIT, otherwise
// it is legal to e.g. haul the load of "K = key(kvs,idx);" outside of
// this loop (since failed CAS ops have no memory ordering semantics).
int dummy = DUMMY_VOLATILE;
continue;
}
// Key slot was not null, there exists a Key here
// We need a volatile-read here to preserve happens-before semantics on
// newly inserted Keys. If the Key body was written just before inserting
// into the table a Key-compare here might read the uninitialized Key body.
// Annoyingly this means we have to volatile-read before EACH key compare.
newkvs = chm._newkvs; // VOLATILE READ before key compare
if( K == key )
break; // Got it!
// get and put must have the same key lookup logic! Lest 'get' give
// up looking too soon.
//topmap._reprobes.add(1);
if( ++reprobe_cnt >= reprobe_limit(len) || // too many probes or
K == TOMBSTONE ) { // found a TOMBSTONE key, means no more keys
// We simply must have a new table to do a 'put'. At this point a
// 'get' will also go to the new table (if any). We do not need
// to claim a key slot (indeed, we cannot find a free one to claim!).
newkvs = chm.resize(topmap,kvs);
if( expVal != null ) topmap.help_copy(newkvs); // help along an existing copy
return putIfMatch0(topmap, newkvs, key, putval, expVal);
}
idx = (idx+1)&(len-1); // Reprobe!
} // End of spinning till we get a Key slot
while ( true ) { // Spin till we insert a value
// ---
// Found the proper Key slot, now update the matching Value slot. We
// never put a null, so Value slots monotonically move from null to
// not-null (deleted Values use Tombstone). Thus if 'V' is null we
// fail this fast cutout and fall into the check for table-full.
if( putval == V ) return V; // Fast cutout for no-change
// See if we want to move to a new table (to avoid high average re-probe
// counts). We only check on the initial set of a Value from null to
// not-null (i.e., once per key-insert). Of course we got a 'free' check
// of newkvs once per key-compare (not really free, but paid-for by the
// time we get here).
if( newkvs == null && // New table-copy already spotted?
// Once per fresh key-insert check the hard way
((V == null && chm.tableFull(reprobe_cnt,len)) ||
// Or we found a Prime, but the JMM allowed reordering such that we
// did not spot the new table (very rare race here: the writing
// thread did a CAS of _newkvs then a store of a Prime. This thread
// reads the Prime, then reads _newkvs - but the read of Prime was so
// delayed (or the read of _newkvs was so accelerated) that they
// swapped and we still read a null _newkvs. The resize call below
// will do a CAS on _newkvs forcing the read.
V instanceof Prime) ) {
newkvs = chm.resize(topmap, kvs); // Force the new table copy to start
}
// See if we are moving to a new table.
// If so, copy our slot and retry in the new table.
if( newkvs != null ) {
return putIfMatch0(topmap, chm.copy_slot_and_check(topmap, kvs, idx, expVal), key, putval, expVal);
}
// ---
// We are finally prepared to update the existing table
assert !(V instanceof Prime);
// Must match old, and we do not? Then bail out now. Note that either V
// or expVal might be TOMBSTONE. Also V can be null, if we've never
// inserted a value before. expVal can be null if we are called from
// copy_slot.
if( expVal != NO_MATCH_OLD && // Do we care about expected-Value at all?
V != expVal && // No instant match already?
(expVal != MATCH_ANY || V == TOMBSTONE || V == null) &&
!(V==null && expVal == TOMBSTONE) && // Match on null/TOMBSTONE combo
(expVal == null || !expVal.equals(V)) ) { // Expensive equals check at the last
return (V == null) ? TOMBSTONE : V; // Do not update!
}
// Actually change the Value in the Key,Value pair
if( CAS_val(kvs, idx, V, putval ) ) break;
// CAS failed
// Because we have no witness, we do not know why it failed.
// Indeed, by the time we look again the value under test might have flipped
// a thousand times and now be the expected value (despite the CAS failing).
// Check for the never-succeed condition of a Prime value and jump to any
// nested table, or else just re-run.
// We would not need this load at all if CAS returned the value on which
// the CAS failed (AKA witness). The new CAS semantics are supported via
// VarHandle in JDK9.
V = val(kvs,idx); // Get new value
// If a Prime'd value got installed, we need to re-run the put on the
// new table. Otherwise we lost the CAS to another racing put.
if( V instanceof Prime )
return putIfMatch0(topmap, chm.copy_slot_and_check(topmap, kvs, idx, expVal), key, putval, expVal);
// Simply retry from the start.
// NOTE: need the fence, since otherwise 'val(kvs,idx)' load could be hoisted
// out of loop.
int dummy = DUMMY_VOLATILE;
}
// CAS succeeded - we did the update!
// Both normal put's and table-copy calls putIfMatch, but table-copy
// does not (effectively) increase the number of live k/v pairs.
if( expVal != null ) {
// Adjust sizes - a striped counter
if( (V == null || V == TOMBSTONE) && putval != TOMBSTONE ) chm._size.add( 1);
if( !(V == null || V == TOMBSTONE) && putval == TOMBSTONE ) chm._size.add(-1);
}
// We won; we know the update happened as expected.
return (V==null && expVal!=null) ? TOMBSTONE : V;
}
// --- help_copy ---------------------------------------------------------
// Help along an existing resize operation. This is just a fast cut-out
// wrapper, to encourage inlining for the fast no-copy-in-progress case. We
// always help the top-most table copy, even if there are nested table
// copies in progress.
private final Object[] help_copy( Object[] helper ) {
// Read the top-level KVS only once. We'll try to help this copy along,
// even if it gets promoted out from under us (i.e., the copy completes
// and another KVS becomes the top-level copy).
Object[] topkvs = _kvs;
CHM topchm = chm(topkvs);
if( topchm._newkvs == null ) return helper; // No copy in-progress
topchm.help_copy_impl(this,topkvs,false);
return helper;
}
// --- CHM -----------------------------------------------------------------
// The control structure for the NonBlockingIdentityHashMap
private static final class CHM {
// Size in active K,V pairs
private final ConcurrentAutoTable _size;
public int size () { return (int)_size.get(); }
// ---
// These next 2 fields are used in the resizing heuristics, to judge when
// it is time to resize or copy the table. Slots is a count of used-up
// key slots, and when it nears a large fraction of the table we probably
// end up reprobing too much. Last-resize-milli is the time since the
// last resize; if we are running back-to-back resizes without growing
// (because there are only a few live keys but many slots full of dead
// keys) then we need a larger table to cut down on the churn.
// Count of used slots, to tell when table is full of dead unusable slots
private final ConcurrentAutoTable _slots;
public int slots() { return (int)_slots.get(); }
// ---
// New mappings, used during resizing.
// The 'new KVs' array - created during a resize operation. This
// represents the new table being copied from the old one. It's the
// volatile variable that is read as we cross from one table to the next,
// to get the required memory orderings. It monotonically transits from
// null to set (once).
volatile Object[] _newkvs;
private static final AtomicReferenceFieldUpdater _newkvsUpdater =
AtomicReferenceFieldUpdater.newUpdater(CHM.class,Object[].class, "_newkvs");
// Set the _next field if we can.
boolean CAS_newkvs( Object[] newkvs ) {
while( _newkvs == null )
if( _newkvsUpdater.compareAndSet(this,null,newkvs) )
return true;
return false;
}
// Sometimes many threads race to create a new very large table. Only 1
// wins the race, but the losers all allocate a junk large table with
// hefty allocation costs. Attempt to control the overkill here by
// throttling attempts to create a new table. I cannot really block here
// (lest I lose the non-blocking property) but late-arriving threads can
// give the initial resizing thread a little time to allocate the initial
// new table. The Right Long Term Fix here is to use array-lets and
// incrementally create the new very large array. In C I'd make the array
// with malloc (which would mmap under the hood) which would only eat
// virtual-address and not real memory - and after Somebody wins then we
// could in parallel initialize the array. Java does not allow
// un-initialized array creation (especially of ref arrays!).
volatile long _resizers; // count of threads attempting an initial resize
private static final AtomicLongFieldUpdater _resizerUpdater =
AtomicLongFieldUpdater.newUpdater(CHM.class, "_resizers");
// ---
// Simple constructor
CHM( ConcurrentAutoTable size ) {
_size = size;
_slots= new ConcurrentAutoTable();
}
// --- tableFull ---------------------------------------------------------
// Heuristic to decide if this table is too full, and we should start a
// new table. Note that if a 'get' call has reprobed too many times and
// decided the table must be full, then always the estimate_sum must be
// high and we must report the table is full. If we do not, then we might
// end up deciding that the table is not full and inserting into the
// current table, while a 'get' has decided the same key cannot be in this
// table because of too many reprobes. The invariant is:
// slots.estimate_sum >= max_reprobe_cnt >= reprobe_limit(len)
private final boolean tableFull( int reprobe_cnt, int len ) {
return
// Do the cheap check first: we allow some number of reprobes always
reprobe_cnt >= REPROBE_LIMIT &&
// More expensive check: see if the table is > 1/4 full.
_slots.estimate_get() >= reprobe_limit(len);
}
// --- resize ------------------------------------------------------------
// Resizing after too many probes. "How Big???" heuristics are here.
// Callers will (not this routine) will 'help_copy' any in-progress copy.
// Since this routine has a fast cutout for copy-already-started, callers
// MUST 'help_copy' lest we have a path which forever runs through
// 'resize' only to discover a copy-in-progress which never progresses.
private final Object[] resize( NonBlockingIdentityHashMap topmap, Object[] kvs) {
assert chm(kvs) == this;
// Check for resize already in progress, probably triggered by another thread
Object[] newkvs = _newkvs; // VOLATILE READ
if( newkvs != null ) // See if resize is already in progress
return newkvs; // Use the new table already
// No copy in-progress, so start one. First up: compute new table size.
int oldlen = len(kvs); // Old count of K,V pairs allowed
int sz = size(); // Get current table count of active K,V pairs
int newsz = sz; // First size estimate
// Heuristic to determine new size. We expect plenty of dead-slots-with-keys
// and we need some decent padding to avoid endless reprobing.
if( sz >= (oldlen>>2) ) { // If we are >25% full of keys then...
newsz = oldlen<<1; // Double size
if( sz >= (oldlen>>1) ) // If we are >50% full of keys then...
newsz = oldlen<<2; // Double double size
}
// This heuristic in the next 2 lines leads to a much denser table
// with a higher reprobe rate
//if( sz >= (oldlen>>1) ) // If we are >50% full of keys then...
// newsz = oldlen<<1; // Double size
// Last (re)size operation was very recent? Then double again; slows
// down resize operations for tables subject to a high key churn rate.
long tm = System.currentTimeMillis();
long q=0;
if( newsz <= oldlen && // New table would shrink or hold steady?
tm <= topmap._last_resize_milli+10000 && // Recent resize (less than 1 sec ago)
(q=_slots.estimate_get()) >= (sz<<1) ) // 1/2 of keys are dead?
newsz = oldlen<<1; // Double the existing size
// Do not shrink, ever
if( newsz < oldlen ) newsz = oldlen;
// Convert to power-of-2
int log2;
for( log2=MIN_SIZE_LOG; (1<>20/*megs*/;
if( r >= 2 && megs > 0 ) { // Already 2 guys trying; wait and see
newkvs = _newkvs; // Between dorking around, another thread did it
if( newkvs != null ) // See if resize is already in progress
return newkvs; // Use the new table already
// TODO - use a wait with timeout, so we'll wakeup as soon as the new table
// is ready, or after the timeout in any case.
//synchronized( this ) { wait(8*megs); } // Timeout - we always wakeup
// For now, sleep a tad and see if the 2 guys already trying to make
// the table actually get around to making it happen.
try { Thread.sleep(8l*megs); } catch( Exception e ) { }
}
// Last check, since the 'new' below is expensive and there is a chance
// that another thread slipped in a new thread while we ran the heuristic.
newkvs = _newkvs;
if( newkvs != null ) // See if resize is already in progress
return newkvs; // Use the new table already
// Double size for K,V pairs, add 1 for CHM
newkvs = new Object[((1< _copyIdxUpdater =
AtomicLongFieldUpdater.newUpdater(CHM.class, "_copyIdx");
// Work-done reporting. Used to efficiently signal when we can move to
// the new table. From 0 to len(oldkvs) refers to copying from the old
// table to the new.
volatile long _copyDone= 0;
static private final AtomicLongFieldUpdater _copyDoneUpdater =
AtomicLongFieldUpdater.newUpdater(CHM.class, "_copyDone");
// --- help_copy_impl ----------------------------------------------------
// Help along an existing resize operation. We hope its the top-level
// copy (it was when we started) but this CHM might have been promoted out
// of the top position.
private final void help_copy_impl( NonBlockingIdentityHashMap topmap, Object[] oldkvs, boolean copy_all ) {
assert chm(oldkvs) == this;
Object[] newkvs = _newkvs;
assert newkvs != null; // Already checked by caller
int oldlen = len(oldkvs); // Total amount to copy
final int MIN_COPY_WORK = Math.min(oldlen,1024); // Limit per-thread work
// ---
int panic_start = -1;
int copyidx=-9999; // Fool javac to think it's initialized
while( _copyDone < oldlen ) { // Still needing to copy?
// Carve out a chunk of work. The counter wraps around so every
// thread eventually tries to copy every slot repeatedly.
// We "panic" if we have tried TWICE to copy every slot - and it still
// has not happened. i.e., twice some thread somewhere claimed they
// would copy 'slot X' (by bumping _copyIdx) but they never claimed to
// have finished (by bumping _copyDone). Our choices become limited:
// we can wait for the work-claimers to finish (and become a blocking
// algorithm) or do the copy work ourselves. Tiny tables with huge
// thread counts trying to copy the table often 'panic'.
if( panic_start == -1 ) { // No panic?
copyidx = (int)_copyIdx;
while( copyidx < (oldlen<<1) && // 'panic' check
!_copyIdxUpdater.compareAndSet(this,copyidx,copyidx+MIN_COPY_WORK) )
copyidx = (int)_copyIdx; // Re-read
if( !(copyidx < (oldlen<<1)) ) // Panic!
panic_start = copyidx; // Record where we started to panic-copy
}
// We now know what to copy. Try to copy.
int workdone = 0;
for( int i=0; i 0 ) // Report work-done occasionally
copy_check_and_promote( topmap, oldkvs, workdone );// See if we can promote
//for( int i=0; i 0 ) {
while( !_copyDoneUpdater.compareAndSet(this,copyDone,copyDone+workdone) ) {
copyDone = _copyDone; // Reload, retry
assert (copyDone+workdone) <= oldlen;
}
}
// Check for copy being ALL done, and promote. Note that we might have
// nested in-progress copies and manage to finish a nested copy before
// finishing the top-level copy. We only promote top-level copies.
if( copyDone+workdone == oldlen && // Ready to promote this table?
topmap._kvs == oldkvs && // Looking at the top-level table?
// Attempt to promote
topmap.CAS_kvs(oldkvs,_newkvs) ) {
topmap._last_resize_milli = System.currentTimeMillis(); // Record resize time for next check
}
}
// --- copy_slot ---------------------------------------------------------
// Copy one K/V pair from oldkvs[i] to newkvs. Returns true if we can
// confirm that we set an old-table slot to TOMBPRIME, and only returns after
// updating the new table. We need an accurate confirmed-copy count so
// that we know when we can promote (if we promote the new table too soon,
// other threads may 'miss' on values not-yet-copied from the old table).
// We don't allow any direct updates on the new table, unless they first
// happened to the old table - so that any transition in the new table from
// null to not-null must have been from a copy_slot (or other old-table
// overwrite) and not from a thread directly writing in the new table.
private boolean copy_slot( NonBlockingIdentityHashMap topmap, int idx, Object[] oldkvs, Object[] newkvs ) {
// Blindly set the key slot from null to TOMBSTONE, to eagerly stop
// fresh put's from inserting new values in the old table when the old
// table is mid-resize. We don't need to act on the results here,
// because our correctness stems from box'ing the Value field. Slamming
// the Key field is a minor speed optimization.
Object key;
while( (key=key(oldkvs,idx)) == null )
CAS_key(oldkvs,idx, null, TOMBSTONE);
// ---
// Prevent new values from appearing in the old table.
// Box what we see in the old table, to prevent further updates.
Object oldval = val(oldkvs,idx); // Read OLD table
while( !(oldval instanceof Prime) ) {
final Prime box = (oldval == null || oldval == TOMBSTONE) ? TOMBPRIME : new Prime(oldval);
if( CAS_val(oldkvs,idx,oldval,box) ) { // CAS down a box'd version of oldval
// If we made the Value slot hold a TOMBPRIME, then we both
// prevented further updates here but also the (absent)
// oldval is vacuously available in the new table. We
// return with true here: any thread looking for a value for
// this key can correctly go straight to the new table and
// skip looking in the old table.
if( box == TOMBPRIME )
return true;
// Otherwise we boxed something, but it still needs to be
// copied into the new table.
oldval = box; // Record updated oldval
break; // Break loop; oldval is now boxed by us
}
oldval = val(oldkvs,idx); // Else try, try again
}
if( oldval == TOMBPRIME ) return false; // Copy already complete here!
// ---
// Copy the value into the new table, but only if we overwrite a null.
// If another value is already in the new table, then somebody else
// wrote something there and that write is happens-after any value that
// appears in the old table.
Object old_unboxed = ((Prime)oldval)._V;
assert old_unboxed != TOMBSTONE;
putIfMatch0(topmap, newkvs, key, old_unboxed, null);
// ---
// Finally, now that any old value is exposed in the new table, we can
// forever hide the old-table value by slapping a TOMBPRIME down. This
// will stop other threads from uselessly attempting to copy this slot
// (i.e., it's a speed optimization not a correctness issue).
while( oldval != TOMBPRIME && !CAS_val(oldkvs,idx,oldval,TOMBPRIME) )
oldval = val(oldkvs,idx);
return oldval != TOMBPRIME; // True if we slammed the TOMBPRIME down
} // end copy_slot
} // End of CHM
// --- Snapshot ------------------------------------------------------------
// The main class for iterating over the NBHM. It "snapshots" a clean
// view of the K/V array.
private class SnapshotV implements Iterator, Enumeration {
final Object[] _sskvs;
public SnapshotV() {
while( true ) { // Verify no table-copy-in-progress
Object[] topkvs = _kvs;
CHM topchm = chm(topkvs);
if( topchm._newkvs == null ) { // No table-copy-in-progress
// The "linearization point" for the iteration. Every key in this
// table will be visited, but keys added later might be skipped or
// even be added to a following table (also not iterated over).
_sskvs = topkvs;
break;
}
// Table copy in-progress - so we cannot get a clean iteration. We
// must help finish the table copy before we can start iterating.
topchm.help_copy_impl(NonBlockingIdentityHashMap.this,topkvs,true);
}
// Warm-up the iterator
next();
}
int length() { return len(_sskvs); }
Object key(int idx) { return NonBlockingIdentityHashMap.key(_sskvs,idx); }
private int _idx; // Varies from 0-keys.length
private Object _nextK, _prevK; // Last 2 keys found
private TypeV _nextV, _prevV; // Last 2 values found
public boolean hasNext() { return _nextV != null; }
public TypeV next() {
// 'next' actually knows what the next value will be - it had to
// figure that out last go-around lest 'hasNext' report true and
// some other thread deleted the last value. Instead, 'next'
// spends all its effort finding the key that comes after the
// 'next' key.
if( _idx != 0 && _nextV == null ) throw new NoSuchElementException();
_prevK = _nextK; // This will become the previous key
_prevV = _nextV; // This will become the previous value
_nextV = null; // We have no more next-key
// Attempt to set <_nextK,_nextV> to the next K,V pair.
// _nextV is the trigger: stop searching when it is != null
while( _idx, but the JDK always removes by key, even when the value has changed.
removeKey();
}
public TypeV nextElement() { return next(); }
public boolean hasMoreElements() { return hasNext(); }
}
/** Returns an enumeration of the values in this table.
* @return an enumeration of the values in this table
* @see #values() */
public Enumeration elements() { return new SnapshotV(); }
// --- values --------------------------------------------------------------
/** Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are reflected
* in the collection, and vice-versa. The collection supports element
* removal, which removes the corresponding mapping from this map, via the
* Iterator.remove, Collection.remove,
* removeAll, retainAll, and clear operations.
* It does not support the add or addAll operations.
*
* The view's iterator is a "weakly consistent" iterator that
* will never throw {@link ConcurrentModificationException}, and guarantees
* to traverse elements as they existed upon construction of the iterator,
* and may (but is not guaranteed to) reflect any modifications subsequent
* to construction. */
@Override
public Collection values() {
return new AbstractCollection() {
@Override public void clear ( ) { NonBlockingIdentityHashMap.this.clear ( ); }
@Override public int size ( ) { return NonBlockingIdentityHashMap.this.size ( ); }
@Override public boolean contains( Object v ) { return NonBlockingIdentityHashMap.this.containsValue(v); }
@Override public Iterator iterator() { return new SnapshotV(); }
};
}
// --- keySet --------------------------------------------------------------
private class SnapshotK implements Iterator, Enumeration {
final SnapshotV _ss;
public SnapshotK() { _ss = new SnapshotV(); }
public void remove() { _ss.removeKey(); }
public TypeK next() { _ss.next(); return (TypeK)_ss._prevK; }
public boolean hasNext() { return _ss.hasNext(); }
public TypeK nextElement() { return next(); }
public boolean hasMoreElements() { return hasNext(); }
}
/** Returns an enumeration of the keys in this table.
* @return an enumeration of the keys in this table
* @see #keySet() */
public Enumeration keys() { return new SnapshotK(); }
/** Returns a {@link Set} view of the keys contained in this map. The set
* is backed by the map, so changes to the map are reflected in the set,
* and vice-versa. The set supports element removal, which removes the
* corresponding mapping from this map, via the Iterator.remove,
* Set.remove, removeAll, retainAll, and
* clear operations. It does not support the add or
* addAll operations.
*
* The view's iterator is a "weakly consistent" iterator that
* will never throw {@link ConcurrentModificationException}, and guarantees
* to traverse elements as they existed upon construction of the iterator,
* and may (but is not guaranteed to) reflect any modifications subsequent
* to construction. */
@Override
public Set keySet() {
return new AbstractSet () {
@Override public void clear ( ) { NonBlockingIdentityHashMap.this.clear ( ); }
@Override public int size ( ) { return NonBlockingIdentityHashMap.this.size ( ); }
@Override public boolean contains( Object k ) { return NonBlockingIdentityHashMap.this.containsKey(k); }
@Override public boolean remove ( Object k ) { return NonBlockingIdentityHashMap.this.remove (k) != null; }
@Override public Iterator iterator() { return new SnapshotK(); }
};
}
// --- entrySet ------------------------------------------------------------
// Warning: Each call to 'next' in this iterator constructs a new NBHMEntry.
private class NBHMEntry extends AbstractEntry {
NBHMEntry( final TypeK k, final TypeV v ) { super(k,v); }
public TypeV setValue(final TypeV val) {
if( val == null ) throw new NullPointerException();
_val = val;
return put(_key, val);
}
}
private class SnapshotE implements Iterator> {
final SnapshotV _ss;
public SnapshotE() { _ss = new SnapshotV(); }
public void remove() {
// NOTE: it would seem logical that entry removal will semantically mean removing the matching pair , but
// the JDK always removes by key, even when the value has changed.
_ss.removeKey();
}
public Map.Entry next() { _ss.next(); return new NBHMEntry((TypeK)_ss._prevK,_ss._prevV); }
public boolean hasNext() { return _ss.hasNext(); }
}
/** Returns a {@link Set} view of the mappings contained in this map. The
* set is backed by the map, so changes to the map are reflected in the
* set, and vice-versa. The set supports element removal, which removes
* the corresponding mapping from the map, via the
* Iterator.remove, Set.remove, removeAll,
* retainAll, and clear operations. It does not support
* the add or addAll operations.
*
* The view's iterator is a "weakly consistent" iterator
* that will never throw {@link ConcurrentModificationException},
* and guarantees to traverse elements as they existed upon
* construction of the iterator, and may (but is not guaranteed to)
* reflect any modifications subsequent to construction.
*
*
Warning: the iterator associated with this Set
* requires the creation of {@link java.util.Map.Entry} objects with each
* iteration. The {@link NonBlockingIdentityHashMap} does not normally create or
* using {@link java.util.Map.Entry} objects so they will be created soley
* to support this iteration. Iterating using {@link Map#keySet} or {@link
* Map##values} will be more efficient.
*/
@Override
public Set> entrySet() {
return new AbstractSet>() {
@Override public void clear ( ) { NonBlockingIdentityHashMap.this.clear( ); }
@Override public int size ( ) { return NonBlockingIdentityHashMap.this.size ( ); }
@Override public boolean remove( final Object o ) {
if( !(o instanceof Map.Entry)) return false;
final Map.Entry e = (Map.Entry)o;
return NonBlockingIdentityHashMap.this.remove(e.getKey(), e.getValue());
}
@Override public boolean contains(final Object o) {
if( !(o instanceof Map.Entry)) return false;
final Map.Entry e = (Map.Entry)o;
TypeV v = get(e.getKey());
return v != null && v.equals(e.getValue());
}
@Override public Iterator> iterator() { return new SnapshotE(); }
};
}
// --- writeObject -------------------------------------------------------
// Write a NBHM to a stream
private void writeObject(java.io.ObjectOutputStream s) throws IOException {
s.defaultWriteObject(); // Nothing to write
for( Object K : keySet() ) {
final Object V = get(K); // Do an official 'get'
s.writeObject(K); // Write the pair
s.writeObject(V);
}
s.writeObject(null); // Sentinel to indicate end-of-data
s.writeObject(null);
}
// --- readObject --------------------------------------------------------
// Read a CHM from a stream
private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException {
s.defaultReadObject(); // Read nothing
initialize(MIN_SIZE);
for(;;) {
final TypeK K = (TypeK) s.readObject();
final TypeV V = (TypeV) s.readObject();
if( K == null ) break;
put(K,V); // Insert with an offical put
}
}
} // End NonBlockingIdentityHashMap class