Class MpmcAtomicUnpaddedArrayQueue<E>

  • All Implemented Interfaces:
    java.lang.Iterable<E>, java.util.Collection<E>, java.util.Queue<E>, IndexedQueueSizeUtil.IndexedQueue, MessagePassingQueue<E>, QueueProgressIndicators, SupportsIterator

    public class MpmcAtomicUnpaddedArrayQueue<E>
    extends MpmcAtomicUnpaddedArrayQueueL3Pad<E>
    NOTE: This class was automatically generated by org.jctools.queues.atomic.unpadded.JavaParsingAtomicUnpaddedArrayQueueGenerator which can found in the jctools-build module. The original source file is MpmcArrayQueue.java. 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 Detail

      • MAX_LOOK_AHEAD_STEP

        public static final int MAX_LOOK_AHEAD_STEP
      • lookAheadStep

        private final int lookAheadStep
    • Constructor Detail

      • MpmcAtomicUnpaddedArrayQueue

        public MpmcAtomicUnpaddedArrayQueue​(int capacity)
    • Method Detail

      • 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
      • 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,
                                     int mask,
                                     java.util.concurrent.atomic.AtomicLongArray sBuffer,
                                     long expectedSeq)
      • 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