#!/usr/bin/perl -w

use strict;
use warnings;

use forks         qw( async );
use forks::shared qw( );

use Thread::Queue qw( );

my $MAX_WORKERS = 4;
my @SERVERS     = qw( server1 server2 server3 );

sub process_request {
   my ($server) = @_;
   return qx{/usr/bin/ssh $server /bin/date 2>/dev/null};
}

sub process_response {
   my ($server, $response) = @_;
   print("$server: $response");
}

{
   my $req = Thread::Queue->new();
   my $res = Thread::Queue->new();

   my $num_workers = $MAX_WORKERS < @SERVERS
      ? $MAX_WORKERS
      : @SERVERS;

   $req->enqueue(@SERVERS);

   my @threads;
   for (1..$num_workers) {
      push @threads, async {
         while (my $server = $req->dequeue()) {
            $res->enqueue( [ $server, process_request($server) ] );
         }
      };
      $req->enqueue(undef);
   }

   process_response(@{$res->dequeue()})
      for 0..$#SERVERS;

   $_->join() for @threads;
}

[download]

The problem with this method is that it fails badly if a child fails abnormally.

Since nothing is added to $req once the children start getting created, you can simplify the code slightly by skipping $req->enqueue(undef); and changing $req->dequeue() to $req->dequeue_nb().

In reply to Re: Forking children operate sequentially?! by ikegami
in thread Forking children operate sequentially?! by darwinscusp

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