EDU.oswego.cs.dl.util.concurrent
Class WaitFreeQueue

java.lang.Object
  EDU.oswego.cs.dl.util.concurrent.WaitFreeQueue

All Implemented Interfaces:

Channel, Puttable, Takable

public class WaitFreeQueue
extends java.lang.Object
implements Channel

A wait-free linked list based queue implementation.

While this class conforms to the full Channel interface, only the put and poll methods are useful in most applications. Because the queue does not support blocking operations, take relies on spin-loops, which can be extremely wasteful.

This class is adapted from the algorithm described in Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue Algorithms by Maged M. Michael and Michael L. Scott. This implementation is not strictly wait-free since it relies on locking for basic atomicity and visibility requirements. Locks can impose unbounded waits, although this should not be a major practical concern here since each lock is held for the duration of only a few statements. (However, the overhead of using so many locks can make it less attractive than other Channel implementations on JVMs where locking operations are very slow.)

See Also:

BoundedLinkedQueue,

[ Introduction to this package. ]

Nested Class Summary

protected static class

WaitFreeQueue.Node List nodes for Queue

Field Summary

protected WaitFreeQueue.Node	head Head of list is always a dummy node
protected WaitFreeQueue.Node	tail Pointer to last node on list
protected java.lang.Object	tailLock Lock for simulating CAS for tail field

Constructor Summary

WaitFreeQueue()

Method Summary

protected boolean	CASHead(WaitFreeQueue.Node oldHead, WaitFreeQueue.Node newHead) Simulate CAS for head field, using 'this' lock
protected boolean	CASTail(WaitFreeQueue.Node oldTail, WaitFreeQueue.Node newTail) Simulate CAS for tail field
protected java.lang.Object	extract() Main dequeue algorithm, called by poll, take.
boolean	offer(java.lang.Object x, long msecs) Place item in channel only if it can be accepted within msecs milliseconds.
java.lang.Object	peek() Return, but do not remove object at head of Channel, or null if it is empty.
java.lang.Object	poll(long msecs) Spin until poll returns a non-null value or time elapses.
void	put(java.lang.Object x) Place item in the channel, possibly waiting indefinitely until it can be accepted.
java.lang.Object	take() Spin until poll returns a non-null value.

Methods inherited from class java.lang.Object

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

Field Detail

head

protected volatile WaitFreeQueue.Node head

Head of list is always a dummy node

tail

protected volatile WaitFreeQueue.Node tail

Pointer to last node on list

tailLock

protected final java.lang.Object tailLock

Lock for simulating CAS for tail field

Constructor Detail

WaitFreeQueue

public WaitFreeQueue()

Method Detail

CASHead

protected boolean CASHead(WaitFreeQueue.Node oldHead,
                          WaitFreeQueue.Node newHead)

Simulate CAS for head field, using 'this' lock

CASTail

protected boolean CASTail(WaitFreeQueue.Node oldTail,
                          WaitFreeQueue.Node newTail)

Simulate CAS for tail field

put

public void put(java.lang.Object x)
         throws java.lang.InterruptedException

Description copied from interface: Channel

Place item in the channel, possibly waiting indefinitely until it can be accepted. Channels implementing the BoundedChannel subinterface are generally guaranteed to block on puts upon reaching capacity, but other implementations may or may not block.

Specified by:

put in interface Channel

Specified by:

put in interface Puttable

Parameters:

x - the element to be inserted. Should be non-null.

Throws:

java.lang.InterruptedException - if the current thread has been interrupted at a point at which interruption is detected, in which case the element is guaranteed not to be inserted. Otherwise, on normal return, the element is guaranteed to have been inserted.

offer

public boolean offer(java.lang.Object x,
                     long msecs)
              throws java.lang.InterruptedException

Description copied from interface: Channel

Place item in channel only if it can be accepted within msecs milliseconds. The time bound is interpreted in a coarse-grained, best-effort fashion.

Specified by:

offer in interface Channel

Specified by:

offer in interface Puttable

Parameters:

x - the element to be inserted. Should be non-null.

msecs - the number of milliseconds to wait. If less than or equal to zero, the method does not perform any timed waits, but might still require access to a synchronization lock, which can impose unbounded delay if there is a lot of contention for the channel.

Returns:

true if accepted, else false

Throws:

java.lang.InterruptedException - if the current thread has been interrupted at a point at which interruption is detected, in which case the element is guaranteed not to be inserted (i.e., is equivalent to a false return).

extract

protected java.lang.Object extract()
                            throws java.lang.InterruptedException

Main dequeue algorithm, called by poll, take.

Throws:

java.lang.InterruptedException

peek

public java.lang.Object peek()

Description copied from interface: Channel

Return, but do not remove object at head of Channel, or null if it is empty.

Specified by:

peek in interface Channel

take

public java.lang.Object take()
                      throws java.lang.InterruptedException

Spin until poll returns a non-null value. You probably don't want to call this method. A Thread.sleep(0) is performed on each iteration as a heuristic to reduce contention. If you would rather use, for example, an exponential backoff, you could manually set this up using poll.

Specified by:

take in interface Channel

Specified by:

take in interface Takable

Returns:

some item from the channel. Different implementations may guarantee various properties (such as FIFO) about that item

Throws:

java.lang.InterruptedException - if the current thread has been interrupted at a point at which interruption is detected, in which case state of the channel is unchanged.

poll

public java.lang.Object poll(long msecs)
                      throws java.lang.InterruptedException

Spin until poll returns a non-null value or time elapses. if msecs is positive, a Thread.sleep(0) is performed on each iteration as a heuristic to reduce contention.

Specified by:

poll in interface Channel

Specified by:

poll in interface Takable

Parameters:

msecs - the number of milliseconds to wait. If less than or equal to zero, the operation does not perform any timed waits, but might still require access to a synchronization lock, which can impose unbounded delay if there is a lot of contention for the channel.

Returns:

some item, or null if the channel is empty.

Throws:

java.lang.InterruptedException - if the current thread has been interrupted at a point at which interruption is detected, in which case state of the channel is unchanged (i.e., equivalent to a null return).