org.jctools.queues

Class SpscLinkedQueue<E>

java.lang.Object

java.util.AbstractCollection<E>

java.util.AbstractQueue<E>

org.jctools.queues.SpscLinkedQueue<E>

Type Parameters:

E -

All Implemented Interfaces:

Iterable<E>, Collection<E>, Queue<E>, MessagePassingQueue<E>

public class SpscLinkedQueue<E>
extends AbstractQueue<E>

This is a weakened version 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. As this is an SPSC we have no need for XCHG, an ordered store is enough.

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 and 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

Constructor and Description
SpscLinkedQueue()

Method Summary

Modifier and Type	Method and 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.
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()
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.
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)
String	toString()

Methods inherited from class java.util.AbstractQueue

add, addAll, clear, element, remove

Methods inherited from class java.util.AbstractCollection

contains, containsAll, remove, removeAll, retainAll, toArray, toArray

Methods inherited from class java.lang.Object

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

Methods inherited from interface org.jctools.queues.MessagePassingQueue

clear

Methods inherited from interface java.util.Collection

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

Methods inherited from interface java.lang.Iterable

forEach

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

SpscLinkedQueue

public SpscLinkedQueue()

Method Detail

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 a SINGLE thread.
Offer allocates a new node (holding the offered value) and:

1. Sets that node as the producerNode.next
2. Sets the new node as the producerNode

From this follows that producerNode.next is always null and for all other nodes node.next is not null.

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 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 empty.
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

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

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.

iterator

public final Iterator<E> iterator()

Specified by:

iterator in interface Iterable<E>

Specified by:

iterator in interface Collection<E>

Specified by:

iterator in class AbstractCollection<E>

toString

public String toString()

Overrides:

toString in class 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 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:

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 Collection<E>

Specified by:

isEmpty in interface MessagePassingQueue<E>

Overrides:

isEmpty in class 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()