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Filename : pool_reuse.pl
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#!/usr/bin/perl

use strict;
use warnings;

use threads 1.39;
use threads::shared;
use Thread::Queue;

### Global Variables ###

# Maximum working threads
my $MAX_THREADS = 10;

# Flag to inform all threads that application is terminating
my $TERM :shared = 0;

# Threads add their ID to this queue when they are ready for work
# Also, when app terminates a -1 is added to this queue
my $IDLE_QUEUE = Thread::Queue->new();


### Signal Handling ###

# Gracefully terminate application on ^C or command line 'kill'
$SIG{'INT'} = $SIG{'TERM'} =
    sub {
        print(">>> Terminating <<<\n");
        $TERM = 1;
        # Add -1 to head of idle queue to signal termination
        $IDLE_QUEUE->insert(0, -1);
    };


### Main Processing Section ###
MAIN:
{
    ### INITIALIZE ###

    # Thread work queues referenced by thread ID
    my %work_queues;

    # Create the thread pool
    for (1..$MAX_THREADS) {
        # Create a work queue for a thread
        my $work_q = Thread::Queue->new();

        # Create the thread, and give it the work queue
        my $thr = threads->create('worker', $work_q);

        # Remember the thread's work queue
        $work_queues{$thr->tid()} = $work_q;
    }


    ### DO WORK ###

    # Manage the thread pool until signalled to terminate
    while (! $TERM) {
        # Wait for an available thread
        my $tid = $IDLE_QUEUE->dequeue();

        # Check for termination condition
        last if ($tid < 0);

        # Give the thread some work to do
        my $work = 5 + int(rand(10));
        $work_queues{$tid}->enqueue($work);
    }


    ### CLEANING UP ###

    # Signal all threads that there is no more work
    $work_queues{$_}->enqueue(-1) foreach keys(%work_queues);

    # Wait for all the threads to finish
    $_->join() foreach threads->list();
}

print("Done\n");
exit(0);


### Thread Entry Point Subroutines ###

# A worker thread
sub worker
{
    my ($work_q) = @_;

    # This thread's ID
    my $tid = threads->tid();

    # Work loop
    do {
        # Indicate that were are ready to do work
        printf("Idle     -> %2d\n", $tid);
        $IDLE_QUEUE->enqueue($tid);

        # Wait for work from the queue
        my $work = $work_q->dequeue();

        # If no more work, exit
        last if ($work < 0);

        # Do some work while monitoring $TERM
        printf("            %2d <- Working\n", $tid);
        while (($work > 0) && ! $TERM) {
            $work -= sleep($work);
        }

        # Loop back to idle state if not told to terminate
    } while (! $TERM);

    # All done
    printf("Finished -> %2d\n", $tid);
}


__END__

=head1 NAME

pool_reuse.pl - Simple 'threads' example with a reusable thread pool

=head1 DESCRIPTION

A simplistic example illustrating the following:

=over

=item * Creation of a pool of reusable threads

=item * Threads use a queue to indicate readiness for handling work

=item * Sending work to threads using queues

=item * Interrupting a threaded program

=item * Cleaning up threads before terminating

=back

=head1 SEE ALSO

L<threads>, L<threads::shared>, and L<Thread::Queue>

=head1 AUTHOR

Jerry D. Hedden, S<E<lt>jdhedden AT cpan DOT orgE<gt>>

=head1 COPYRIGHT AND LICENSE

Copyright 2006 - 2009 Jerry D. Hedden. All rights reserved.

This program is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.

=cut