-
Notifications
You must be signed in to change notification settings - Fork 2.3k
/
MarshlandPool.java
180 lines (161 loc) · 5.78 KB
/
MarshlandPool.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
/*
* Copyright (c) 2002-2017 "Neo Technology,"
* Network Engine for Objects in Lund AB [http://neotechnology.com]
*
* This file is part of Neo4j.
*
* Neo4j is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package org.neo4j.collection.pool;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import static java.util.Collections.newSetFromMap;
/**
* A concurrent pool that attempts to use thread-local pooling (puddles!) rather than a single big pool of objects to
* lower contention. Falls back to a delegate pool if no local object can be claimed. This means this can be used as
* a wrapper around a contended pool to alleviate contention in cases where threads generally claim and release one object
* at a time.
*/
public class MarshlandPool<T> implements Pool<T>
{
/*
* This is a somewhat complicated class. What it does is to keep a single-slot local pool for each thread that
* uses it, to allow very rapid claim calls that don't need to communicate with other threads. However, this is
* dangerous, since pooled objects may then be lost when threads die.
*
* To mitigate this, the local slots are tracked by phantom references, which allows us to use a reference queue
* to find objects that used to "belong" to now-dead threads.
*
* So, our algo for claiming is:
* - Check thread local for available object.
* - If none found, check the reference queue
* - If none found, use the delegate pool.
*/
private final Pool<T> pool;
private final ThreadLocal<LocalSlot<T>> puddle = new ThreadLocal<LocalSlot<T>>()
{
@Override
protected LocalSlot<T> initialValue()
{
LocalSlot<T> localSlot = new LocalSlot<>( objectsFromDeadThreads );
slotReferences.add( localSlot.phantomReference );
return localSlot;
}
};
// Used to reclaim objects from dead threads
private final Set<LocalSlotReference> slotReferences =
newSetFromMap( new ConcurrentHashMap<LocalSlotReference, Boolean>() );
private final ReferenceQueue<LocalSlot<T>> objectsFromDeadThreads = new ReferenceQueue<>();
public MarshlandPool( Pool<T> delegatePool )
{
this.pool = delegatePool;
}
@Override
public T acquire()
{
// Try and get it from the thread local
LocalSlot<T> localSlot = puddle.get();
T object = localSlot.object;
if ( object != null )
{
localSlot.set( null );
return object;
}
// Try the reference queue, containing objects from dead threads
@SuppressWarnings( "unchecked" )
LocalSlotReference<T> slotReference = (LocalSlotReference) objectsFromDeadThreads.poll();
if ( slotReference != null && slotReference.object != null )
{
slotReferences.remove( slotReference );
return slotReference.object;
}
// Fall back to the delegate pool
return pool.acquire();
}
@Override
public void release( T obj )
{
// Return it locally if possible
LocalSlot<T> localSlot = puddle.get();
if ( localSlot.object == null )
{
localSlot.set( obj );
}
else
{
// Fall back to the delegate pool
pool.release( obj );
}
}
/**
* Dispose of all objects in this pool, releasing them back to the delegate pool
*/
@SuppressWarnings( "unchecked" )
public void disposeAll()
{
for ( LocalSlotReference slotReference : slotReferences )
{
LocalSlot<T> slot = (LocalSlot) slotReference.get();
if ( slot != null )
{
T obj = slot.object;
if ( obj != null )
{
slot.set( null );
pool.release( obj );
}
}
}
for ( LocalSlotReference<T> reference = (LocalSlotReference) objectsFromDeadThreads.poll();
reference != null;
reference = (LocalSlotReference) objectsFromDeadThreads.poll() )
{
T instance = reference.object;
if ( instance != null )
{
pool.release( instance );
}
}
}
/**
* This is used to trigger the GC to notify us whenever the thread local has been garbage collected.
*/
private static class LocalSlotReference<T> extends WeakReference<LocalSlot>
{
private T object;
private LocalSlotReference( LocalSlot referent, ReferenceQueue<? super LocalSlot> q )
{
super( referent, q );
}
}
/**
* Container for the "puddle", the small local pool each thread keeps.
*/
private static class LocalSlot<T>
{
private T object;
private final LocalSlotReference phantomReference;
LocalSlot( ReferenceQueue<LocalSlot<T>> referenceQueue )
{
phantomReference = new LocalSlotReference( this, referenceQueue );
}
public void set( T obj )
{
phantomReference.object = obj;
this.object = obj;
}
}
}