Package org.jctools.queues

Class MpscLinkedQueue<E>

java.lang.Object

java.util.AbstractCollection<E>

java.util.AbstractQueue<E>

org.jctools.queues.MpscLinkedQueue<E>

Type Parameters:

E - the type of elements in this queue

All Implemented Interfaces:

MessagePassingQueue<E>

Direct Known Subclasses:

MpscLinkedQueue7, MpscLinkedQueue8

public abstract class MpscLinkedQueue<E>
extends java.util.AbstractQueue<E>

This is a direct 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).

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

Nested Class Summary

Nested classes/interfaces inherited from interface org.jctools.queues.MessagePassingQueue

MessagePassingQueue.Consumer<T>, MessagePassingQueue.ExitCondition, MessagePassingQueue.Supplier<T>, MessagePassingQueue.WaitStrategy

Field Summary

Fields

Modifier and Type	Field	Description
protected static long	C_NODE_OFFSET
protected org.jctools.queues.LinkedQueueNode<E>	consumerNode
protected static long	P_NODE_OFFSET
protected org.jctools.queues.LinkedQueueNode<E>	producerNode

Fields inherited from interface org.jctools.queues.MessagePassingQueue

UNBOUNDED_CAPACITY

Constructor Summary

Constructors

Modifier	Constructor	Description
protected	MpscLinkedQueue()

Method Summary

Modifier and Type	Method	Description
int	capacity()
protected boolean	casProducerNode(org.jctools.queues.LinkedQueueNode<E> expect, org.jctools.queues.LinkedQueueNode<E> newValue)
int	drain(MessagePassingQueue.Consumer<E> c)	Remove all available item from the queue and hand to consume.
int	drain(MessagePassingQueue.Consumer<E> c, int limit)	Remove up to limit elements from the queue and hand to consume.
void	drain(MessagePassingQueue.Consumer<E> c, MessagePassingQueue.WaitStrategy wait, MessagePassingQueue.ExitCondition exit)	Remove elements from the queue and hand to consume forever.
int	fill(MessagePassingQueue.Supplier<E> s)	Stuff the queue with elements from the supplier.
int	fill(MessagePassingQueue.Supplier<E> s, int limit)	Stuff the queue with up to limit elements from the supplier.
void	fill(MessagePassingQueue.Supplier<E> s, MessagePassingQueue.WaitStrategy wait, MessagePassingQueue.ExitCondition exit)	Stuff the queue with elements from the supplier forever.
protected E	getSingleConsumerNodeValue(org.jctools.queues.LinkedQueueNode<E> currConsumerNode, org.jctools.queues.LinkedQueueNode<E> nextNode)
boolean	isEmpty()	This method's accuracy is subject to concurrent modifications happening as the observation is carried out.
java.util.Iterator<E>	iterator()
protected org.jctools.queues.LinkedQueueNode<E>	lpConsumerNode()
protected org.jctools.queues.LinkedQueueNode<E>	lpProducerNode()
protected org.jctools.queues.LinkedQueueNode<E>	lvConsumerNode()
protected org.jctools.queues.LinkedQueueNode<E>	lvProducerNode()
static <E> MpscLinkedQueue<E>	newMpscLinkedQueue()	Construct the implementation based on availability of getAndSet intrinsic.
protected org.jctools.queues.LinkedQueueNode<E>	newNode()
protected org.jctools.queues.LinkedQueueNode<E>	newNode(E e)
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.
E	peek()	Called from the consumer thread subject to the restrictions appropriate to the implementation and according to the Queue.peek() interface.
E	poll()	Called from the consumer thread subject to the restrictions appropriate to the implementation and according to the Queue.poll() interface.
boolean	relaxedOffer(E e)	Called from a producer thread subject to the restrictions appropriate to the implementation.
E	relaxedPeek()	Called from the consumer thread subject to the restrictions appropriate to the implementation.
E	relaxedPoll()	Called from the consumer thread subject to the restrictions appropriate to the implementation.
boolean	remove(java.lang.Object o)
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.
protected void	spConsumerNode(org.jctools.queues.LinkedQueueNode<E> newValue)
protected void	spProducerNode(org.jctools.queues.LinkedQueueNode<E> newValue)
java.lang.String	toString()
protected abstract org.jctools.queues.LinkedQueueNode<E>	xchgProducerNode(org.jctools.queues.LinkedQueueNode<E> nextNode)

Methods inherited from class java.util.AbstractCollection

contains, containsAll, removeAll, retainAll, toArray, toArray

Methods inherited from class java.util.AbstractQueue

add, addAll, clear, element, remove

Methods inherited from interface java.util.Collection

contains, containsAll, equals, hashCode, parallelStream, removeAll, removeIf, retainAll, spliterator, stream, toArray, toArray

Methods inherited from interface java.lang.Iterable

forEach

Methods inherited from interface org.jctools.queues.MessagePassingQueue

clear

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

C_NODE_OFFSET

protected static final long C_NODE_OFFSET

consumerNode

protected org.jctools.queues.LinkedQueueNode<E> consumerNode

P_NODE_OFFSET

protected static final long P_NODE_OFFSET

producerNode

protected org.jctools.queues.LinkedQueueNode<E> producerNode

Constructor Detail

MpscLinkedQueue

protected MpscLinkedQueue()

Method Detail

newMpscLinkedQueue

public static <E> MpscLinkedQueue<E> newMpscLinkedQueue()

Construct the implementation based on availability of getAndSet intrinsic.

Returns:

the right queue for you!

offer

public final 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:

MessagePassingQueue.offer(Object), Queue.offer(java.lang.Object)

poll

public final 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 reads the next node from the consumerNode and:

1. If it is null, the queue is assumed empty (though it might not be).
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:

MessagePassingQueue.poll(), Queue.poll()

peek

public final 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

remove

public final boolean remove(java.lang.Object o)

This method is only safe to call from the (single) consumer thread, and is subject to best effort when racing with producers.

Overrides:

remove in class java.util.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.

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.

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.

xchgProducerNode

protected abstract org.jctools.queues.LinkedQueueNode<E> xchgProducerNode(org.jctools.queues.LinkedQueueNode<E> nextNode)

iterator

public final java.util.Iterator<E> iterator()

Specified by:

iterator in class java.util.AbstractCollection<E>

toString

public java.lang.String toString()

Overrides:

toString in class java.util.AbstractCollection<E>

newNode

protected final org.jctools.queues.LinkedQueueNode<E> newNode()

newNode

protected final org.jctools.queues.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 MessagePassingQueue<E>

Specified by:

size in class java.util.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:

Collection.size()

isEmpty

public final 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 MessagePassingQueue<E>

Overrides:

isEmpty in class java.util.AbstractCollection<E>

Returns:

true if empty, false otherwise

See Also:

MessagePassingQueue.isEmpty()

getSingleConsumerNodeValue

protected E getSingleConsumerNodeValue(org.jctools.queues.LinkedQueueNode<E> currConsumerNode,
                                       org.jctools.queues.LinkedQueueNode<E> nextNode)

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)

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.

Returns:

the number of polled elements

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.

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.

capacity

public int capacity()

Returns:

the capacity of this queue or MessagePassingQueue.UNBOUNDED_CAPACITY if not bounded

spConsumerNode

protected final void spConsumerNode(org.jctools.queues.LinkedQueueNode<E> newValue)

lvConsumerNode

protected final org.jctools.queues.LinkedQueueNode<E> lvConsumerNode()

lpConsumerNode

protected final org.jctools.queues.LinkedQueueNode<E> lpConsumerNode()

spProducerNode

protected final void spProducerNode(org.jctools.queues.LinkedQueueNode<E> newValue)

lvProducerNode

protected final org.jctools.queues.LinkedQueueNode<E> lvProducerNode()

casProducerNode

protected final boolean casProducerNode(org.jctools.queues.LinkedQueueNode<E> expect,
                                        org.jctools.queues.LinkedQueueNode<E> newValue)

lpProducerNode

protected final org.jctools.queues.LinkedQueueNode<E> lpProducerNode()