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Better implementation of AttributeMap

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Motivation:
The old DefaultAttributeMap impl did more synchronization then needed.

Modifications:
* Rewrite DefaultAttributeMap to not use IdentityHashMap and synchronization on the map directly. The new impl uses a combination of AtomicReferenceArray and synchronization per chain (linked-list). Also access the first Attribute per bucket can be done without any synchronization at all and just uses atomic operations. This should fit for most use-cases pretty weel.

Result:
Synchronization is per linked-list and the first entry can even be added via atomic operation.
This commit is contained in:
Norman Maurer 2014-05-13 14:23:23 +02:00
parent 795507aa7b
commit 8d1ac97bd9
1 changed files with 84 additions and 25 deletions
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109
common/src/main/java/io/netty/util/DefaultAttributeMap.java
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@ -17,65 +17,111 @@ package io.netty.util;
import io.netty.util.internal.PlatformDependent;
import java.util.IdentityHashMap;
import java.util.Map;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.atomic.AtomicReferenceArray;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
/**
* Default {@link AttributeMap} implementation which use simple synchronization to keep the memory overhead
* Default {@link AttributeMap} implementation which use simple synchronization per bucket to keep the memory overhead
* as low as possible.
*/
public class DefaultAttributeMap implements AttributeMap {
@SuppressWarnings("rawtypes")
private static final AtomicReferenceFieldUpdater<DefaultAttributeMap, Map> updater;
private static final AtomicReferenceFieldUpdater<DefaultAttributeMap, AtomicReferenceArray> updater;
static {
@SuppressWarnings("rawtypes")
AtomicReferenceFieldUpdater<DefaultAttributeMap, Map> referenceFieldUpdater =
PlatformDependent.newAtomicReferenceFieldUpdater(DefaultAttributeMap.class, "map");
AtomicReferenceFieldUpdater<DefaultAttributeMap, AtomicReferenceArray> referenceFieldUpdater =
PlatformDependent.newAtomicReferenceFieldUpdater(DefaultAttributeMap.class, "attributes");
if (referenceFieldUpdater == null) {
referenceFieldUpdater = AtomicReferenceFieldUpdater.newUpdater(DefaultAttributeMap.class, Map.class, "map");
referenceFieldUpdater = AtomicReferenceFieldUpdater
.newUpdater(DefaultAttributeMap.class, AtomicReferenceArray.class, "attributes");
}
updater = referenceFieldUpdater;
}
private static final int BUCKET_SIZE = 4;
private static final int MASK = BUCKET_SIZE - 1;
// Initialize lazily to reduce memory consumption; updated by AtomicReferenceFieldUpdater above.
@SuppressWarnings("UnusedDeclaration")
private volatile Map<AttributeKey<?>, Attribute<?>> map;
private volatile AtomicReferenceArray<DefaultAttribute<?>> attributes;
@SuppressWarnings({ "unchecked", "rawtypes" })
@Override
public <T> Attribute<T> attr(AttributeKey<T> key) {
Map<AttributeKey<?>, Attribute<?>> map = this.map;
if (map == null) {
if (key == null) {
throw new NullPointerException("key");
}
AtomicReferenceArray<DefaultAttribute<?>> attributes = this.attributes;
if (attributes == null) {
// Not using ConcurrentHashMap due to high memory consumption.
map = new IdentityHashMap<AttributeKey<?>, Attribute<?>>(2);
if (!updater.compareAndSet(this, null, map)) {
map = this.map;
attributes = new AtomicReferenceArray<DefaultAttribute<?>>(BUCKET_SIZE);
if (!updater.compareAndSet(this, null, attributes)) {
attributes = this.attributes;
}
}
synchronized (map) {
@SuppressWarnings("unchecked")
Attribute<T> attr = (Attribute<T>) map.get(key);
if (attr == null) {
attr = new DefaultAttribute<T>(map, key);
map.put(key, attr);
int i = index(key);
DefaultAttribute<?> head = attributes.get(i);
if (head == null) {
// No head exists yet which means we may be able to add the attribute without synchronization and just
// use compare and set. At worst we need to fallback to synchronization
head = new DefaultAttribute(key);
if (attributes.compareAndSet(i, null, head)) {
// we were able to add it so return the head right away
return (Attribute<T>) head;
} else {
head = attributes.get(i);
}
}
synchronized (head) {
DefaultAttribute<?> curr = head;
for (;;) {
if (!curr.removed && curr.key == key) {
return (Attribute<T>) curr;
}
DefaultAttribute<?> next = curr.next;
if (next == null) {
DefaultAttribute<T> attr = new DefaultAttribute<T>(head, key);
curr.next = attr;
attr.prev = curr;
}
}
return attr;
}
}
private static int index(AttributeKey<?> key) {
return key.id() & MASK;
}
@SuppressWarnings("serial")
private static final class DefaultAttribute<T> extends AtomicReference<T> implements Attribute<T> {
private static final long serialVersionUID = -2661411462200283011L;
private final Map<AttributeKey<?>, Attribute<?>> map;
// The head of the linked-list this attribute belongs to, which may be itself
private final DefaultAttribute<?> head;
private final AttributeKey<T> key;
DefaultAttribute(Map<AttributeKey<?>, Attribute<?>> map, AttributeKey<T> key) {
this.map = map;
// Double-linked list to prev and next node to allow fast removal
private DefaultAttribute<?> prev;
private DefaultAttribute<?> next;
// Will be set to true one the attribute is removed via getAndRemove() or remove()
private volatile boolean removed;
DefaultAttribute(DefaultAttribute<?> head, AttributeKey<T> key) {
this.head = head;
this.key = key;
}
DefaultAttribute(AttributeKey<T> key) {
head = this;
this.key = key;
}
@ -97,6 +143,7 @@ public class DefaultAttributeMap implements AttributeMap {
@Override
public T getAndRemove() {
removed = true;
T oldValue = getAndSet(null);
remove0();
return oldValue;
@ -104,13 +151,25 @@ public class DefaultAttributeMap implements AttributeMap {
@Override
public void remove() {
removed = true;
set(null);
remove0();
}
private void remove0() {
synchronized (map) {
map.remove(key);
synchronized (head) {
// We only update the linked-list structure if prev != null because if it is null this
// DefaultAttribute acts also as head. The head must never be removed completely and just be
// marked as removed as all synchronization is done on the head itself for each bucket.
// The head itself will be GC'ed once the DefaultAttributeMap is GC'ed. So at most 5 heads will
// be removed lazy as the array size is 5.
if (prev != null) {
prev.next = next;
if (next != null) {
next.prev = prev;
}
}
}
}
}