Package edu.rit.pj

Class IntegerStrideForLoop

java.lang.Object

edu.rit.pj.ParallelConstruct

edu.rit.pj.ParallelForLoop

edu.rit.pj.IntegerStrideForLoop

public abstract class IntegerStrideForLoop
extends ParallelForLoop

Class IntegerStrideForLoop is the abstract base class for one variation of a parallel for loop that is executed inside a ParallelRegion. The loop index data type is int. The loop stride is explicitly specified.

To execute a parallel for loop, create a ParallelRegion object; create an instance of a concrete subclass of class IntegerStrideForLoop; and pass this instance to the parallel region's execute() method. Either every parallel team thread must call the parallel region's execute() method with identical arguments, or every thread must not call the execute() method. You can do all this using an anonymous inner class; for example:

     new ParallelRegion()
         {
         . . .
         public void run()
             {
             . . .
             execute (0, 98, 2, new IntegerStrideForLoop()
                 {
                 // Thread local variable declarations
                 . . .
                 public void start()
                     {
                     // Per-thread pre-loop initialization code
                     . . .
                     }
                 public void run (int first, int last, int stride)
                     {
                     // Loop code
                     . . .
                     }
                 public void finish()
                     {
                     // Per-thread post-loop finalization code
                     . . .
                     }
                 });
             }
         . . .
         }
 

The parallel region's execute() method does the following. Each parallel team thread calls the parallel for loop's start() method once before beginning any loop iterations. The range of loop indexes is divided into "chunks" and the chunks are apportioned among the threads, in a manner determined by the parallel for loop's schedule as returned by the schedule() method. Each thread repeatedly calls the parallel for loop's run() method, passing in a different chunk on each call, until all the chunks assigned to that thread have been performed. When a thread has finished calling run(), the thread calls the parallel for loop's finish() method. Then the thread waits at an implicit barrier. When all the threads have reached the barrier, the execute() method returns.

Note that each parallel team thread actually creates its own instance of the parallel for loop class and passes that instance to the parallel region's execute() method. Thus, any fields declared in the parallel for loop class will not be shared by all the threads, but instead will be private to each thread.

The start() method is intended for performing per-thread initialization before starting the loop iterations. If no such initialization is needed, omit the start() method.

The run() method contains the code for the loop. The first and last indexes for a chunk of loop iterations are passed in as arguments. The loop stride, which is always positive, is also explicitly specified as an argument. The parallel for loop's run() method must be coded this way:

     public void run (int first, int last, int stride)
         {
         for (int i = first; i <= last; i += stride)
             {
             // Loop body code
             . . .
             }
         }
 

with the loop indexes running from first to last inclusive and increasing by stride on each iteration.

The finish() method is intended for performing per-thread finalization after finishing the loop iterations. If no such finalization is needed, omit the finish() method.

Sometimes a portion of a parallel for loop has to be executed sequentially in the order of the loop indexes, while the rest of the parallel for loop can be executed concurrently. For example, the loop body is performing some computation that can be executed in parallel for different loop indexes, but the results of each computation must be written to a file sequentially in the order of the loop indexes. The ordered() method is provided for this purpose. A call to the ordered() method may appear once in the parallel for loop's run() method, like so:

     public void run (int first, int last, int stride)
         {
         for (int i = first; i <= last; i += stride)
             {
             // This portion executed concurrently
             . . .
             ordered (new ParallelSection()
                 {
                 public void run()
                     {
                     // This portion executed sequentially
                     // in the order of the loop indexes
                     . . .
                     }
                 });
             // This portion executed concurrently again
             . . .
             }
         }
 

When called, the ordered() method waits until the ordered() method has been called and has returned in all loop iterations prior to the current loop iteration. Then the ordered() method calls the given parallel section's run() method. When the parallel section's run() method returns, the ordered() method returns. If the parallel section's run() method throws an exception, the ordered() method throws that same exception.

It is possible to stop a parallel for loop using the stopLoop() method, like this:

     public void run (int first, int last, int stride)
         {
         for (int i = first; i <= last; i += stride)
             {
             // Loop body
             . . .
             if (/*time to stop the loop*/)
                 {
                 stopLoop();
                 break;
                 }
             // More loop body
             . . .
             }
         }
 

Once stopLoop() is called, after each parallel team thread finishes executing its current chunk of iterations, each thread will execute no further chunks and will proceed to finish the parallel for loop. Note well that stopping a parallel for loop is not the same as executing a break statement in a regular for loop. The parallel for loop does not stop until each thread, including the thread that called stopLoop(), has finished its current chunk of iterations. Thus, depending on the parallel for loop's schedule, additional iterations may be executed after stopLoop() is called. (The break statement in the above example causes the thread that called stopLoop() to finish its chunk of iterations early.)

Normally, at the end of the parallel for loop, the parallel team threads wait for each other at a barrier. To eliminate this barrier wait, include BarrierAction.NO_WAIT in the execute() method call:

     new ParallelRegion()
         {
         . . .
         public void run()
             {
             . . .
             execute (0, 98, 2, new IntegerStrideForLoop()
                 {
                 . . .
                 },
             BarrierAction.NO_WAIT);
             . . .
             }
         }
 

To execute a section of code in a single thread as part of the barrier synchronization, include an instance of class BarrierAction in the execute() method call. The barrier action object's run() method contains the code to be executed in a single thread while the other threads wait:

     new ParallelRegion()
         {
         . . .
         public void run()
             {
             . . .
             execute (0, 98, 2, new IntegerStrideForLoop()
                 {
                 . . .
                 },
             new BarrierAction()
                 {
                 public void run()
                     {
                     // Single-threaded code goes here
                     . . .
                     }
                 });
             . . .
             }
         }
 

For further information, see class BarrierAction.

If the parallel for loop's start(), run(), or finish() method throws an exception in one of the threads, then that thread executes no further code in the loop, and the parallel region's execute() method throws that same exception in that thread. Furthermore, the other threads in the parallel team also execute no further code in the loop after finishing their current chunks. Thus, if one thread throws an exception, the whole parallel for loop exits with some (perhaps none) of the iterations unperformed.

Version:

11-Nov-2007

Author:

Alan Kaminsky

Constructor Summary

Constructors

Constructor	Description
IntegerStrideForLoop()	Construct a new parallel for loop.

Method Summary

Modifier and Type	Method	Description
void	finish()	Perform per-thread finalization actions after finishing the loop iterations.
final void	ordered(ParallelSection theSection)	Execute the given section of code in order of the loop indexes.
abstract void	run(int first, int last, int stride)	Execute one chunk of iterations of this parallel for loop.
IntegerSchedule	schedule()	Determine this parallel for loop's schedule.
void	start()	Perform per-thread initialization actions before starting the loop iterations.
final void	stopLoop()	Stop this parallel for loop.

Methods inherited from class edu.rit.pj.ParallelConstruct

getThreadCount, getThreadIndex, isExecutingInParallel, region, team

Methods inherited from class java.lang.Object

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

Constructor Details

IntegerStrideForLoop

public IntegerStrideForLoop()

Construct a new parallel for loop.

Method Details

schedule

public IntegerSchedule schedule()

Determine this parallel for loop's schedule. The schedule determines how the loop iterations are apportioned among the parallel team threads. For further information, see class IntegerSchedule.

The schedule() method may be overridden in a subclass to return the desired schedule. If not overridden, the default is a runtime schedule (see IntegerSchedule.runtime()).

Returns:

Schedule for this parallel for loop.

start

public void start()
           throws Exception

Perform per-thread initialization actions before starting the loop iterations.

The start() method may be overridden in a subclass. If not overridden, the start() method does nothing.

Throws:

Exception - The start() method may throw any exception.

Exception - if any.

run

public abstract void run(int first,
 int last,
 int stride)
                  throws Exception

Execute one chunk of iterations of this parallel for loop. The run() method must perform the loop body for indexes first through last inclusive, increasing the loop index by stride after each iteration.

The run() method must be overridden in a subclass.

Parameters:

first - First loop index.

last - Last loop index.

stride - Loop index stride, always positive.

Throws:

Exception - The run() method may throw any exception.

Exception - if any.

finish

public void finish()
            throws Exception

Perform per-thread finalization actions after finishing the loop iterations.

The finish() method may be overridden in a subclass. If not overridden, the finish() method does nothing.

Throws:

Exception - The finish() method may throw any exception.

Exception - if any.

ordered

public final void ordered(ParallelSection theSection)
                   throws Exception

Execute the given section of code in order of the loop indexes. A call to the ordered() method may appear in this parallel for loop's run() method. When called, the ordered() method waits until the ordered() method has been called and has returned in all loop iterations prior to the current loop iteration. Then the ordered() method calls the run() method of theParallelSection. When the parallel section's run() method returns, the ordered() method returns. If the parallel section's run() method throws an exception, the ordered() method throws that same exception.

The ordered() method is used when a portion of a parallel for loop has to be executed sequentially in the order of the loop indexes, while the rest of the parallel for loop can be executed concurrently.

Note: Either the ordered() method must be called exactly once during each call of the parallel for loop's run() method, or the ordered() method must not be called at all.

Parameters:

theSection - Parallel section to execute in order.

Throws:

NullPointerException - (unchecked exception) Thrown if theSection is null.

IllegalStateException - (unchecked exception) Thrown if no parallel team is executing this parallel for loop.

Exception - Thrown if theSection's run() method throws an exception.

Exception - if any.

stopLoop

public final void stopLoop()

Stop this parallel for loop. Once stopLoop() is called, after each parallel team thread finishes executing its current chunk of iterations, each thread will execute no further chunks and will proceed to finish this parallel for loop.

Throws:

IllegalStateException - (unchecked exception) Thrown if no parallel team is executing this parallel for loop.