src/main/java/rx/internal/util/unsafe/MpscLinkedQueue.java
/*
* 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.
*
* Original License: https://github.com/JCTools/JCTools/blob/master/LICENSE
* Original location: https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/MpscLinkedQueue.java
*/
package rx.internal.util.unsafe;
import static rx.internal.util.unsafe.UnsafeAccess.UNSAFE;
import rx.internal.util.SuppressAnimalSniffer;
import rx.internal.util.atomic.LinkedQueueNode;
/**
* This is a direct Java port of the MPSC algorithm as presented <a
* href="http://www.1024cores.net/home/lock-free-algorithms/queues/non-intrusive-mpsc-node-based-queue"> on 1024
* Cores</a> by D. Vyukov. The original has been adapted to Java and it's quirks with regards to memory model and
* layout:
* <ol>
* <li>Use inheritance to ensure no false sharing occurs between producer/consumer node reference fields.
* <li>Use XCHG functionality to the best of the JDK ability (see differences in JDK7/8 implementations).
* </ol>
* The queue is initialized with a stub node which is set to both the producer and consumer node references. From this
* point follow the notes on offer/poll.
*
* @author nitsanw
*
* @param <E>
*/
@SuppressAnimalSniffer
public final class MpscLinkedQueue<E> extends BaseLinkedQueue<E> {
public MpscLinkedQueue() {
consumerNode = new LinkedQueueNode<E>();
xchgProducerNode(consumerNode);// this ensures correct construction: StoreLoad
}
@SuppressWarnings("unchecked")
protected LinkedQueueNode<E> xchgProducerNode(LinkedQueueNode<E> newVal) {
Object oldVal;
do {
oldVal = producerNode;
} while (!UNSAFE.compareAndSwapObject(this, P_NODE_OFFSET, oldVal, newVal));
return (LinkedQueueNode<E>) oldVal;
}
/**
* {@inheritDoc} <br>
* <p>
* IMPLEMENTATION NOTES:<br>
* Offer is allowed from multiple threads.<br>
* Offer allocates a new node and:
* <ol>
* <li>Swaps it atomically with current producer node (only one producer 'wins')
* <li>Sets the new node as the node following from the swapped producer node
* </ol>
* This works because each producer is guaranteed to 'plant' a new node and link the old node. No 2 producers can
* get the same producer node as part of XCHG guarantee.
*
* @see MessagePassingQueue#offer(Object)
* @see java.util.Queue#offer(java.lang.Object)
*/
@Override
public boolean offer(final E nextValue) {
if (nextValue == null) {
throw new NullPointerException("null elements not allowed");
}
final LinkedQueueNode<E> nextNode = new LinkedQueueNode<E>(nextValue);
final LinkedQueueNode<E> prevProducerNode = xchgProducerNode(nextNode);
// Should a producer thread get interrupted here the chain WILL be broken until that thread is resumed
// and completes the store in prev.next.
prevProducerNode.soNext(nextNode); // StoreStore
return true;
}
/**
* {@inheritDoc} <br>
* <p>
* IMPLEMENTATION NOTES:<br>
* Poll is allowed from a SINGLE thread.<br>
* Poll reads the next node from the consumerNode and:
* <ol>
* <li>If it is null, the queue is assumed empty (though it might not be).
* <li>If it is not null set it as the consumer node and return it's now evacuated value.
* </ol>
* This means the consumerNode.value is always null, which is also the starting point for the queue. Because null
* values are not allowed to be offered this is the only node with it's value set to null at any one time.
*
* @see MessagePassingQueue#poll()
* @see java.util.Queue#poll()
*/
@Override
public E poll() {
LinkedQueueNode<E> currConsumerNode = lpConsumerNode(); // don't load twice, it's alright
LinkedQueueNode<E> nextNode = currConsumerNode.lvNext();
if (nextNode != null) {
// we have to null out the value because we are going to hang on to the node
final E nextValue = nextNode.getAndNullValue();
spConsumerNode(nextNode);
return nextValue;
}
else if (currConsumerNode != lvProducerNode()) {
// spin, we are no longer wait free
while ((nextNode = currConsumerNode.lvNext()) == null) { } // NOPMD
// got the next node...
// we have to null out the value because we are going to hang on to the node
final E nextValue = nextNode.getAndNullValue();
consumerNode = nextNode;
return nextValue;
}
return null;
}
@Override
public E peek() {
LinkedQueueNode<E> currConsumerNode = consumerNode; // don't load twice, it's alright
LinkedQueueNode<E> nextNode = currConsumerNode.lvNext();
if (nextNode != null) {
return nextNode.lpValue();
}
else if (currConsumerNode != lvProducerNode()) {
// spin, we are no longer wait free
while ((nextNode = currConsumerNode.lvNext()) == null) { } // NOPMD
// got the next node...
return nextNode.lpValue();
}
return null;
}
}