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

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

All Implemented Interfaces:

Barrier

public class CyclicBarrier
extends java.lang.Object
implements Barrier

A cyclic barrier is a reasonable choice for a barrier in contexts involving a fixed sized group of threads that must occasionally wait for each other. (A Rendezvous better handles applications in which any number of threads meet, n-at-a-time.)

CyclicBarriers use an all-or-none breakage model for failed synchronization attempts: If threads leave a barrier point prematurely because of timeout or interruption, others will also leave abnormally (via BrokenBarrierException), until the barrier is restarted. This is usually the simplest and best strategy for sharing knowledge about failures among cooperating threads in the most common usages contexts of Barriers. This implementation has the property that interruptions among newly arriving threads can cause as-yet-unresumed threads from a previous barrier cycle to return out as broken. This transmits breakage as early as possible, but with the possible byproduct that only some threads returning out of a barrier will realize that it is newly broken. (Others will not realize this until a future cycle.) (The Rendezvous class has a more uniform, but sometimes less desirable policy.)

Barriers support an optional Runnable command that is run once per barrier point.

Sample usage Here is a code sketch of a barrier in a parallel decomposition design.

 class Solver {
   final int N;
   final float[][] data;
   final CyclicBarrier barrier;
   
   class Worker implements Runnable {
      int myRow;
      Worker(int row) { myRow = row; }
      public void run() {
         while (!done()) {
            processRow(myRow);

            try {
              barrier.barrier(); 
            }
            catch (InterruptedException ex) { return; }
            catch (BrokenBarrierException ex) { return; }
         }
      }
   }

   public Solver(float[][] matrix) {
     data = matrix;
     N = matrix.length;
     barrier = new CyclicBarrier(N);
     barrier.setBarrierCommand(new Runnable() {
       public void run() { mergeRows(...); }
     });
     for (int i = 0; i < N; ++i) {
       new Thread(new Worker(i)).start();
     waitUntilDone();
    }
 }
 

[ Introduction to this package. ]

Field Summary

protected java.lang.Runnable	barrierCommand_
protected boolean	broken_
protected int	count_
protected int	parties_
protected int	resets_

Constructor Summary

CyclicBarrier(int parties)
Create a CyclicBarrier for the indicated number of parties, and no command to run at each barrier.

CyclicBarrier(int parties, java.lang.Runnable command)
Create a CyclicBarrier for the indicated number of parties.

Method Summary

int	attemptBarrier(long msecs) Enter barrier and wait at most msecs for the other parties()-1 threads.
int	barrier() Enter barrier and wait for the other parties()-1 threads.
boolean	broken() Returns true if the barrier has been compromised by threads leaving the barrier before a synchronization point (normally due to interruption or timeout).
protected int	doBarrier(boolean timed, long msecs)
int	parties() Return the number of parties that must meet per barrier point.
void	restart() Reset to initial state.
java.lang.Runnable	setBarrierCommand(java.lang.Runnable command) Set the command to run at the point at which all threads reach the barrier.

Methods inherited from class java.lang.Object

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

Field Detail

parties_

protected final int parties_

broken_

protected boolean broken_

barrierCommand_

protected java.lang.Runnable barrierCommand_

count_

protected int count_

resets_

protected int resets_

Constructor Detail

CyclicBarrier

public CyclicBarrier(int parties)

Create a CyclicBarrier for the indicated number of parties, and no command to run at each barrier.

Throws:

java.lang.IllegalArgumentException - if parties less than or equal to zero.

CyclicBarrier

public CyclicBarrier(int parties,
                     java.lang.Runnable command)

Create a CyclicBarrier for the indicated number of parties. and the given command to run at each barrier point.

Throws:

java.lang.IllegalArgumentException - if parties less than or equal to zero.

Method Detail

setBarrierCommand

public java.lang.Runnable setBarrierCommand(java.lang.Runnable command)

Set the command to run at the point at which all threads reach the barrier. This command is run exactly once, by the thread that trips the barrier. The command is not run if the barrier is broken.

Parameters:

command - the command to run. If null, no command is run.

Returns:

the previous command

broken

public boolean broken()

Description copied from interface: Barrier

Returns true if the barrier has been compromised by threads leaving the barrier before a synchronization point (normally due to interruption or timeout). Barrier methods in implementation classes throw throw BrokenBarrierException upon detection of breakage. Implementations may also support some means to clear this status.

Specified by:

broken in interface Barrier

restart

public void restart()

Reset to initial state. Clears both the broken status and any record of waiting threads, and releases all currently waiting threads with indeterminate return status. This method is intended only for use in recovery actions in which it is somehow known that no thread could possibly be relying on the the synchronization properties of this barrier.

parties

public int parties()

Description copied from interface: Barrier

Return the number of parties that must meet per barrier point. The number of parties is always at least 1.

Specified by:

parties in interface Barrier

barrier

public int barrier()
            throws java.lang.InterruptedException,
                   BrokenBarrierException

Enter barrier and wait for the other parties()-1 threads.

Returns:

the arrival index: the number of other parties that were still waiting upon entry. This is a unique value from zero to parties()-1. If it is zero, then the current thread was the last party to hit barrier point and so was responsible for releasing the others.

Throws:

BrokenBarrierException - if any other thread in any previous or current barrier since either creation or the last restart operation left the barrier prematurely due to interruption or time-out. (If so, the broken status is also set.) Threads that are notified to have been interrupted after being released are not considered to have broken the barrier. In all cases, the interruption status of the current thread is preserved, so can be tested by checking Thread.interrupted.

java.lang.InterruptedException - if this thread was interrupted during the barrier, and was the one causing breakage. If so, broken status is also set.

attemptBarrier

public int attemptBarrier(long msecs)
                   throws java.lang.InterruptedException,
                          TimeoutException,
                          BrokenBarrierException

Enter barrier and wait at most msecs for the other parties()-1 threads.

Returns:

if not timed out, the arrival index: the number of other parties that were still waiting upon entry. This is a unique value from zero to parties()-1. If it is zero, then the current thread was the last party to hit barrier point and so was responsible for releasing the others.

Throws:

BrokenBarrierException - if any other thread in any previous or current barrier since either creation or the last restart operation left the barrier prematurely due to interruption or time-out. (If so, the broken status is also set.) Threads that are noticed to have been interrupted after being released are not considered to have broken the barrier. In all cases, the interruption status of the current thread is preserved, so can be tested by checking Thread.interrupted.

java.lang.InterruptedException - if this thread was interrupted during the barrier. If so, broken status is also set.

TimeoutException - if this thread timed out waiting for the barrier. If the timeout occured while already in the barrier, broken status is also set.

doBarrier

protected int doBarrier(boolean timed,
                        long msecs)
                 throws java.lang.InterruptedException,
                        TimeoutException,
                        BrokenBarrierException

Throws:

java.lang.InterruptedException

TimeoutException

BrokenBarrierException