Question

I am completely new to Perl, like absolute newbie. I am trying to develop a system which reads a database and, according to the results, generates a queue which launches another script.

HERE is the source code.

Now the script works as expected, except I have noticed that it doesn’t really do the threads parallel. Whether I use 1 thread or 50 threads, the execution time is the same; 1 thread is even faster.
When I have the script display which thread did what, I see the threads don’t run at the same time, because it will do thread 1, then 2, then 3 etc.

Does anyone know what I did wrong here? Again the script itself works, just not in parallel threads.

Answer 1

You need to learn what semaphores actually are before you start using them. You’ve explicitly told the threads not to run in parallel:

my $s           = Thread::Semaphore->new;
#...

    while ($queue_id_list->pending > 0) { 
        $s->down;
        my $info = $queue_id_list->dequeue_nb;
        if (defined($info)) {
            my @details = split(/#/, $info);
            #my $result = system("./match_name db=user_".$details[0]." id=".$details[1]); 
            # normally the script above would be launched which is a php script run in php-cli and does some database things

            sleep(0.1);

            #print "Thread: ". threads->self->tid. " - Done user: ".$details[0]. " and addressbook id: ". $details[1]."\r\n";
            #print $queue_id_list->pending."\r\n";
        }
        $s->up;
    }

You’ve created a semaphore $s, which by default has a count of 1. Then in the function you’re trying to run, you call $s->down at the start — which decreases the count by 1, or blocks if the count is already <1, and $s->up at the end, which increases the count by 1.

Once a thread calls down, no other threads will run until it calls up again.

You should carefully read the Thread::Semaphore docs, and probably this wikipedia article on semaphores, too.