Class MpscLinkedQueue<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractQueue<E>
org.jctools.queues.MpscLinkedQueue<E>
All Implemented Interfaces:
Iterable<E>, Collection<E>, Queue<E>, MessagePassingQueue<E>
public class MpscLinkedQueue<E> extends AbstractQueue<E>
This is a Java port of the MPSC algorithm as presented on 1024 Cores by D. Vyukov. The original has been adapted to Java and it's quirks with regards to memory model and layout:
  1. Use inheritance to ensure no false sharing occurs between producer/consumer node reference fields.
  2. Use XCHG functionality to the best of the JDK ability (see differences in JDK7/8 impls).
  3. Conform to Queue contract on poll. The original semantics are available via relaxedPoll.
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.

  • Constructor Details

    • MpscLinkedQueue

      public MpscLinkedQueue()

  • Method Details

    • offer

      public boolean offer(E e)
      Called from a producer thread subject to the restrictions appropriate to the implementation and according to the Queue.offer(Object) interface.

      IMPLEMENTATION NOTES:
      Offer is allowed from multiple threads.
      Offer allocates a new node and:

      1. Swaps it atomically with current producer node (only one producer 'wins')
      2. Sets the new node as the node following from the swapped producer node
      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.
      Parameters:
      e - not null, will throw NPE if it is
      Returns:
      true if element was inserted into the queue, false iff full
      See Also:

    • remove

      public boolean remove(Object o)

      This method is only safe to call from the (single) consumer thread, and is subject to best effort when racing with producers. This method is potentially blocking when "bubble"s in the queue are visible.

      Specified by:
      remove in interface Collection<E>
      Overrides:
      remove in class AbstractCollection<E>

    • fill

      public int fill(MessagePassingQueue.Supplier<E> s)
      Description copied from interface: MessagePassingQueue
      Stuff the queue with elements from the supplier. Semantically similar to: 
      while(relaxedOffer(s.get());
      There's no strong commitment to the queue being full at the end of a fill. Called from a producer thread subject to the restrictions appropriate to the implementation.

      Unbounded queues will fill up the queue with a fixed amount rather than fill up to oblivion. WARNING: Explicit assumptions are made with regards to MessagePassingQueue.Supplier.get() make sure you have read and understood these before using this method.

      Returns:
      the number of offered elements

    • fill

      public int fill(MessagePassingQueue.Supplier<E> s, int limit)
      Description copied from interface: MessagePassingQueue
      Stuff the queue with up to limit elements from the supplier. Semantically similar to: 

        for(int i=0; i < limit && relaxedOffer(s.get()); i++);

      There's no strong commitment to the queue being full at the end of a fill. Called from a producer thread subject to the restrictions appropriate to the implementation. WARNING: Explicit assumptions are made with regards to MessagePassingQueue.Supplier.get() make sure you have read and understood these before using this method.

      Returns:
      the number of offered elements

    • fill

      public void fill(MessagePassingQueue.Supplier<E> s, MessagePassingQueue.WaitStrategy wait, MessagePassingQueue.ExitCondition exit)
      Description copied from interface: MessagePassingQueue
      Stuff the queue with elements from the supplier forever. Semantically similar to: 

      
 int idleCounter = 0;
 while (exit.keepRunning()) {
     E e = s.get();
     while (!relaxedOffer(e)) {
         idleCounter = wait.idle(idleCounter);
         continue;
     }
     idleCounter = 0;
 }

      Called from a producer thread subject to the restrictions appropriate to the implementation. The main difference being that implementors MUST assure room in the queue is available BEFORE calling MessagePassingQueue.Supplier.get(). WARNING: Explicit assumptions are made with regards to MessagePassingQueue.Supplier.get() make sure you have read and understood these before using this method.

    • iterator

      public final Iterator<E> iterator()
      Specified by:
      iterator in interface Collection<E>
      Specified by:
      iterator in interface Iterable<E>
      Specified by:
      iterator in class AbstractCollection<E>

    • toString

      public String toString()
      Overrides:
      toString in class AbstractCollection<E>

    • newNode

      protected final LinkedQueueNode<E> newNode()

    • newNode

      protected final LinkedQueueNode<E> newNode(E e)

    • size

      public final int size()
      This method's accuracy is subject to concurrent modifications happening as the size is estimated and as such is a best effort rather than absolute value. For some implementations this method may be O(n) rather than O(1).

      IMPLEMENTATION NOTES:
      This is an O(n) operation as we run through all the nodes and count them.
      The accuracy of the value returned by this method is subject to races with producer/consumer threads. In particular when racing with the consumer thread this method may under estimate the size.

      Specified by:
      size in interface Collection<E>
      Specified by:
      size in interface MessagePassingQueue<E>
      Specified by:
      size in class AbstractCollection<E>
      Returns:
      number of messages in the queue, between 0 and Integer.MAX_VALUE but less or equals to capacity (if bounded).
      See Also:

    • isEmpty

      public boolean isEmpty()
      This method's accuracy is subject to concurrent modifications happening as the observation is carried out.

      IMPLEMENTATION NOTES:
      Queue is empty when producerNode is the same as consumerNode. An alternative implementation would be to observe the producerNode.value is null, which also means an empty queue because only the consumerNode.value is allowed to be null.

      Specified by:
      isEmpty in interface Collection<E>
      Specified by:
      isEmpty in interface MessagePassingQueue<E>
      Overrides:
      isEmpty in class AbstractCollection<E>
      Returns:
      true if empty, false otherwise
      See Also:

    • getSingleConsumerNodeValue

      protected E getSingleConsumerNodeValue(LinkedQueueNode<E> currConsumerNode, LinkedQueueNode<E> nextNode)

    • poll

      public E poll()
      Called from the consumer thread subject to the restrictions appropriate to the implementation and according to the Queue.poll() interface.

      IMPLEMENTATION NOTES:
      Poll is allowed from a SINGLE thread.
      Poll is potentially blocking here as the Queue.poll() does not allow returning null if the queue is not empty. This is very different from the original Vyukov guarantees. See MessagePassingQueue.relaxedPoll() for the original semantics.
      Poll reads consumerNode.next and:

      1. If it is null AND the queue is empty return null, if queue is not empty spin wait for value to become visible.
      2. If it is not null set it as the consumer node and return it's now evacuated value.
      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.
      Returns:
      a message from the queue if one is available, null iff empty
      See Also:

    • peek

      public E peek()
      Called from the consumer thread subject to the restrictions appropriate to the implementation and according to the Queue.peek() interface.

      IMPLEMENTATION NOTES:
      Peek is allowed from a SINGLE thread.
      Peek is potentially blocking here as the Queue.peek() does not allow returning null if the queue is not empty. This is very different from the original Vyukov guarantees. See MessagePassingQueue.relaxedPeek() for the original semantics.
      Poll reads the next node from the consumerNode and:

      1. If it is null AND the queue is empty return null, if queue is not empty spin wait for value to become visible.
      2. If it is not null return it's value.
      Returns:
      a message from the queue if one is available, null iff empty
      See Also:

    • relaxedPoll

      public E relaxedPoll()
      Description copied from interface: MessagePassingQueue
      Called from the consumer thread subject to the restrictions appropriate to the implementation. As opposed to Queue.poll() this method may return null without the queue being empty.
      Returns:
      a message from the queue if one is available, null if unable to poll

    • relaxedPeek

      public E relaxedPeek()
      Description copied from interface: MessagePassingQueue
      Called from the consumer thread subject to the restrictions appropriate to the implementation. As opposed to Queue.peek() this method may return null without the queue being empty.
      Returns:
      a message from the queue if one is available, null if unable to peek

    • relaxedOffer

      public boolean relaxedOffer(E e)
      Description copied from interface: MessagePassingQueue
      Called from a producer thread subject to the restrictions appropriate to the implementation. As opposed to Queue.offer(Object) this method may return false without the queue being full.
      Parameters:
      e - not null, will throw NPE if it is
      Returns:
      true if element was inserted into the queue, false if unable to offer

    • drain

      public int drain(MessagePassingQueue.Consumer<E> c, int limit)
      Description copied from interface: MessagePassingQueue
      Remove up to limit elements from the queue and hand to consume. This should be semantically similar to: 

        M m;
  int i = 0;
  for(;i < limit && (m = relaxedPoll()) != null; i++){
    c.accept(m);
  }
  return i;

      There's no strong commitment to the queue being empty at the end of a drain. Called from a consumer thread subject to the restrictions appropriate to the implementation.

      WARNING: Explicit assumptions are made with regards to MessagePassingQueue.Consumer.accept(T) make sure you have read and understood these before using this method.

      Returns:
      the number of polled elements

    • drain

      public int drain(MessagePassingQueue.Consumer<E> c)
      Description copied from interface: MessagePassingQueue
      Remove all available item from the queue and hand to consume. This should be semantically similar to: 
      M m;
while((m = relaxedPoll()) != null){
c.accept(m);
}
      There's no strong commitment to the queue being empty at the end of a drain. Called from a consumer thread subject to the restrictions appropriate to the implementation.

      WARNING: Explicit assumptions are made with regards to MessagePassingQueue.Consumer.accept(T) make sure you have read and understood these before using this method.

      Returns:
      the number of polled elements

    • drain

      public void drain(MessagePassingQueue.Consumer<E> c, MessagePassingQueue.WaitStrategy wait, MessagePassingQueue.ExitCondition exit)
      Description copied from interface: MessagePassingQueue
      Remove elements from the queue and hand to consume forever. Semantically similar to: 

       int idleCounter = 0;
 while (exit.keepRunning()) {
     E e = relaxedPoll();
     if(e==null){
         idleCounter = wait.idle(idleCounter);
         continue;
     }
     idleCounter = 0;
     c.accept(e);
 }

      Called from a consumer thread subject to the restrictions appropriate to the implementation.

      WARNING: Explicit assumptions are made with regards to MessagePassingQueue.Consumer.accept(T) make sure you have read and understood these before using this method.

    • capacity

      public int capacity()
      Returns:
      the capacity of this queue or MessagePassingQueue.UNBOUNDED_CAPACITY if not bounded