/*
* 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_voltpatches.queues;
/**
* This is a weakened version 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>As this is an SPSC we have no need for XCHG, an ordered store is enough.
* </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>
*/
public class SpscLinkedQueue<E> extends BaseLinkedQueue<E> {
public SpscLinkedQueue() {
spProducerNode(new LinkedQueueNode<E>());
spConsumerNode(producerNode);
consumerNode.soNext(null); // this ensures correct construction: StoreStore
}
/**
* {@inheritDoc} <br>
*
* IMPLEMENTATION NOTES:<br>
* Offer is allowed from a SINGLE thread.<br>
* Offer allocates a new node (holding the offered value) and:
* <ol>
* <li>Sets that node as the producerNode.next
* <li>Sets the new node as the producerNode
* </ol>
* From this follows that producerNode.next is always null and for all other nodes node.next is not null.
*
* @see MessagePassingQueue#offer(Object)
* @see java.util.Queue#offer(java.lang.Object)
*/
@Override
public boolean offer(final E e) {
if (null == e) {
throw new NullPointerException();
}
final LinkedQueueNode<E> nextNode = new LinkedQueueNode<E>(e);
final LinkedQueueNode<E> producerNode = lpProducerNode();
producerNode.soNext(nextNode);
spProducerNode(nextNode);
return true;
}
/**
* {@inheritDoc} <br>
*
* 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 empty.
* <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.
*
*/
@Override
public E poll() {
return relaxedPoll();
}
@Override
public E peek() {
return relaxedPeek();
}
@Override
public int fill(Supplier<E> s) {
long result = 0;// result is a long because we want to have a safepoint check at regular intervals
do {
fill(s, 4096);
result += 4096;
} while (result <= Integer.MAX_VALUE - 4096);
return (int) result;
}
@Override
public int fill(Supplier<E> s, int limit) {
if (limit == 0) return 0;
LinkedQueueNode<E> tail = new LinkedQueueNode<E>(s.get());
final LinkedQueueNode<E> head = tail;
for (int i = 1; i < limit; i++) {
final LinkedQueueNode<E> temp = new LinkedQueueNode<E>(s.get());
tail.soNext(temp);
tail = temp;
}
final LinkedQueueNode<E> oldPNode = lpProducerNode();
oldPNode.soNext(head);
spProducerNode(tail);
return limit;
}
@Override
public void fill(Supplier<E> s, WaitStrategy wait, ExitCondition exit) {
LinkedQueueNode<E> chaserNode = producerNode;
while (exit.keepRunning()) {
for (int i = 0; i < 4096; i++) {
final LinkedQueueNode<E> nextNode = new LinkedQueueNode<E>(s.get());
chaserNode.soNext(nextNode);
chaserNode = nextNode;
this.producerNode = chaserNode;
}
}
}
}