/* * 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.atomic; /** * 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 XCHG functionality provided by AtomicReference (which is better in JDK 8+). * </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 final class MpscLinkedAtomicQueue<E> extends BaseLinkedAtomicQueue<E> { public MpscLinkedAtomicQueue() { super(); LinkedQueueAtomicNode<E> node = new LinkedQueueAtomicNode<E>(); spConsumerNode(node); xchgProducerNode(node);// this ensures correct construction: StoreLoad } /** * {@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 org.jctools_voltpatches.queues.MessagePassingQueue#offer(Object) * @see java.util.Queue#offer(java.lang.Object) */ @Override public final boolean offer(final E e) { if (null == e) { throw new NullPointerException(); } final LinkedQueueAtomicNode<E> nextNode = new LinkedQueueAtomicNode<E>(e); final LinkedQueueAtomicNode<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 org.jctools_voltpatches.queues.MessagePassingQueue#poll() * @see java.util.Queue#poll() */ @Override public final E poll() { LinkedQueueAtomicNode<E> currConsumerNode = lpConsumerNode(); // don't load twice, it's alright LinkedQueueAtomicNode<E> nextNode = currConsumerNode.lvNext(); if (nextNode != null) { return getSingleConsumerNodeValue(currConsumerNode, nextNode); } else if (currConsumerNode != lvProducerNode()) { // spin, we are no longer wait free while((nextNode = currConsumerNode.lvNext()) == null); // got the next node... return getSingleConsumerNodeValue(currConsumerNode, nextNode); } return null; } @Override public final E peek() { LinkedQueueAtomicNode<E> currConsumerNode = lpConsumerNode(); // don't load twice, it's alright LinkedQueueAtomicNode<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); // got the next node... return nextNode.lpValue(); } return null; } }