Class MpmcArrayQueue<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractQueue<E>
org.jctools.queues.ConcurrentCircularArrayQueueL0Pad<E>
org.jctools.queues.ConcurrentCircularArrayQueue<E>
org.jctools.queues.ConcurrentSequencedCircularArrayQueue<E>
org.jctools.queues.MpmcArrayQueueL1Pad<E>
org.jctools.queues.MpmcArrayQueueProducerIndexField<E>
org.jctools.queues.MpmcArrayQueueL2Pad<E>
org.jctools.queues.MpmcArrayQueueConsumerIndexField<E>
org.jctools.queues.MpmcArrayQueueL3Pad<E>
org.jctools.queues.MpmcArrayQueue<E>
All Implemented Interfaces:
Iterable<E>, Collection<E>, Queue<E>, IndexedQueueSizeUtil.IndexedQueue, MessagePassingQueue<E>, QueueProgressIndicators, SupportsIterator
public class MpmcArrayQueue<E> extends MpmcArrayQueueL3Pad<E>
A Multi-Producer-Multi-Consumer queue based on a ConcurrentCircularArrayQueue. This implies that any and all threads may call the offer/poll/peek methods and correctness is maintained.
This implementation follows patterns documented on the package level for False Sharing protection.
The algorithm for offer/poll is an adaptation of the one put forward by D. Vyukov (See here). The original algorithm uses an array of structs which should offer nice locality properties but is sadly not possible in Java (waiting on Value Types or similar). The alternative explored here utilizes 2 arrays, one for each field of the struct. There is a further alternative in the experimental project which uses iteration phase markers to achieve the same algo and is closer structurally to the original, but sadly does not perform as well as this implementation.

Tradeoffs to keep in mind:

  1. Padding for false sharing: counter fields and queue fields are all padded as well as either side of both arrays. We are trading memory to avoid false sharing(active and passive).
  2. 2 arrays instead of one: The algorithm requires an extra array of longs matching the size of the elements array. This is doubling/tripling the memory allocated for the buffer.
  3. Power of 2 capacity: Actual elements buffer (and sequence buffer) is the closest power of 2 larger or equal to the requested capacity.

  • Field Details

    • MAX_LOOK_AHEAD_STEP

      public static final int MAX_LOOK_AHEAD_STEP

    • lookAheadStep

      private final int lookAheadStep

  • Constructor Details

    • MpmcArrayQueue

      public MpmcArrayQueue(int capacity)

  • Method Details

    • offer

      public boolean offer(E e)
      Description copied from interface: MessagePassingQueue
      Called from a producer thread subject to the restrictions appropriate to the implementation and according to the Queue.offer(Object) interface.
      Parameters:
      e - not null, will throw NPE if it is
      Returns:
      true if element was inserted into the queue, false iff full

    • poll

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

      Because return null indicates queue is empty we cannot simply rely on next element visibility for poll and must test producer index when next element is not visible.

      Returns:
      a message from the queue if one is available, null iff empty

    • peek

      public E peek()
      Description copied from interface: MessagePassingQueue
      Called from the consumer thread subject to the restrictions appropriate to the implementation and according to the Queue.peek() interface.
      Returns:
      a message from the queue if one is available, null iff empty

    • 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

    • 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

    • 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

    • drainOneByOne

      private int drainOneByOne(MessagePassingQueue.Consumer<E> c, int limit)

    • 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

    • notAvailable

      private boolean notAvailable(long index, long mask, long[] sBuffer, long expectedSeq)

    • fillOneByOne

      private int fillOneByOne(MessagePassingQueue.Supplier<E> s, int limit)

    • 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

    • 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

    • drain

      public void drain(MessagePassingQueue.Consumer<E> c, MessagePassingQueue.WaitStrategy w, 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.

    • 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.