I am looking for a way to limit the number of concurrent executions of a script to some value x, where [1 <= x <= n ]. Limiting to 1 instance appears to be the easy case, but it seems more difficult to limit it to just some n greater than 1.

Because of the manner in which the process may be launched (a filter launched by another process-and no, not email-related), it is conceivable that several thousand instances could be launched in the span of a few minutes; however, I wish to limit this to a much smaller (and processable) number (such as less than 100). Because of the time involved (and other potential issues), I am trying to avoid solutions such as the creation of a large number of lock files (due to speed issues), or ones susceptible to race-conditions (or the like).

One suggestion I encountered suggested the use of IPC::Semaphore as a way, by allowing processes to update a shared semaphore, and exit if the value it contained exceeded my limit. (I believe I understood that correctly-please correct me if I am wrong, however.) I seem to be having difficulties implementing this, although now it seems to be working for the moment. Does it look as if I have this correct, and can anyone suggest a better way, or any issues they may see with my implementation? 

#!/usr/bin/perl -w

use strict;

use IPC::SysV qw(IPC_PRIVATE S_IRWXU IPC_CREAT);
use IPC::Semaphore;

srand;

my $sem =
  new IPC::Semaphore( 1234, 1, S_IRWXU | IPC_CREAT );

my $semval = $sem->getval(0);
$semval = 0 unless ( defined($semval) );

exit(0) if ( $semval >= 10 );
$sem->setval( 0, ( $semval + 1 ) );

#
# Do work... or in this case, 
# print the value of $semval and sleep for a while
#
print $semval + 1, "\n";
sleep( int( rand(20) + 1 ) );

#
# Decrement the semaphore value
#
$semval = $sem->getval(0);
if ($semval) {
    $sem->setval( 0, ( $semval - 1 ) );
}
else {
    $sem->remove;
}
__END__
#
# Sample test execution (in bash):
#
# index=30
# while [ $index -gt 0 ] 
# do 
#     perl test.pl & 
#     sleep 1 
#     let "index=index-1" 
# done
#

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In reply to Effective use of IPC::Semaphore to limit number of concurrent executions of a scxipt by atcroft

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