hu.akarnokd.rxjava2.internal.queue.SpscArrayQueue Maven / Gradle / Ivy
Show all versions of rxjava2-backport Show documentation
/**
* Copyright 2015 David Karnok and Netflix, Inc.
*
* 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.
*/
/*
* The code was inspired by the similarly named JCTools class:
* https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/atomic
*/
package hu.akarnokd.rxjava2.internal.queue;
import java.util.concurrent.atomic.AtomicLong;
/**
* A Single-Producer-Single-Consumer queue backed by a pre-allocated buffer.
*
* This implementation is a mashup of the Fast Flow
* algorithm with an optimization of the offer method taken from the BQueue algorithm (a variation on Fast
* Flow), and adjusted to comply with Queue.offer semantics with regards to capacity.
* For convenience the relevant papers are available in the resources folder:
* 2010 - Pisa - SPSC Queues on Shared Cache Multi-Core Systems.pdf
* 2012 - Junchang- BQueue- Efficient and Practical Queuing.pdf
* This implementation is wait free.
*
* @param
*/
public final class SpscArrayQueue extends BaseArrayQueue {
/** */
private static final long serialVersionUID = -1296597691183856449L;
private static final Integer MAX_LOOK_AHEAD_STEP = Integer.getInteger("jctools.spsc.max.lookahead.step", 4096);
final AtomicLong producerIndex;
protected long producerLookAhead;
final AtomicLong consumerIndex;
final int lookAheadStep;
public SpscArrayQueue(int capacity) {
super(capacity);
this.producerIndex = new AtomicLong();
this.consumerIndex = new AtomicLong();
lookAheadStep = Math.min(capacity / 4, MAX_LOOK_AHEAD_STEP);
}
@Override
public boolean offer(E e) {
if (null == e) {
throw new NullPointerException("Null is not a valid element");
}
// local load of field to avoid repeated loads after volatile reads
final int mask = this.mask;
final long index = producerIndex.get();
final int offset = calcElementOffset(index, mask);
if (index >= producerLookAhead) {
int step = lookAheadStep;
if (null == lvElement(calcElementOffset(index + step, mask))) {// LoadLoad
producerLookAhead = index + step;
}
else if (null != lvElement(offset)){
return false;
}
}
soProducerIndex(index + 1); // ordered store -> atomic and ordered for size()
soElement(offset, e); // StoreStore
return true;
}
@Override
public E poll() {
final long index = consumerIndex.get();
final int offset = calcElementOffset(index);
// local load of field to avoid repeated loads after volatile reads
final E e = lvElement(offset);// LoadLoad
if (null == e) {
return null;
}
soConsumerIndex(index + 1); // ordered store -> atomic and ordered for size()
soElement(offset, null);// StoreStore
return e;
}
@Override
public E peek() {
return lvElement(calcElementOffset(consumerIndex.get()));
}
@Override
public boolean isEmpty() {
return producerIndex.get() == consumerIndex.get();
}
@Override
public int size() {
/*
* It is possible for a thread to be interrupted or reschedule between the read of the producer and consumer
* indices, therefore protection is required to ensure size is within valid range. In the event of concurrent
* polls/offers to this method the size is OVER estimated as we read consumer index BEFORE the producer index.
*/
long after = lvConsumerIndex();
while (true) {
final long before = after;
final long currentProducerIndex = lvProducerIndex();
after = lvConsumerIndex();
if (before == after) {
return (int) (currentProducerIndex - after);
}
}
}
private void soProducerIndex(long newIndex) {
producerIndex.lazySet(newIndex);
}
private void soConsumerIndex(long newIndex) {
consumerIndex.lazySet(newIndex);
}
private long lvConsumerIndex() {
return consumerIndex.get();
}
private long lvProducerIndex() {
return producerIndex.get();
}
}