makow2 has asked for the wisdom of the Perl Monks concerning the following question:

Hello, Simple code: 
#!/usr/bin/perl

use HTTP::Daemon;
use Parallel::ForkManager;

$daemon = new HTTP::Daemon(LocalPort => 8080, LocalAddr => "127.0.0.1"
+, Listen => 64, ReuseAddr => 1) or die "$!";

$pm = Parallel::ForkManager->new(3);

while (1)
{
    $inputcon = $daemon->accept();

    $pm->start and next; 

    do_client_stuff($inputcon);

    $pm->finish();
}

sub do_client_stuff
{
    my ($inputcon) = @_;

    $request = $inputcon->get_request;

    print $request . "\n";

    $inputcon->send_error(403);
 }

[download]
Almost everything is OK. But the last child process leaves a zombie on the system. I can only do wget once and its enough to make <defunct> process. Is there any solution for this? I don't want to change forkmanager to proc::queue or something. Thanks. Makowik /Sorry for my english/

Replies are listed 'Best First'.
Re: Parallel::ForkManager leaves a zombie process from last child
by runrig (Abbot) on May 30, 2013 at 20:39 UTC
    Already asked and answered on StackOverflow. You just have not accepted the answer. Bad form to cross post and not mention it.

    Summary: Parallel::ForkManager reaps its processes when the number of processes has reached the max and you try to start a new one. Since you are in an infinite loop, there is always the 'last' process that remains unreaped until a new one starts (all of them remain unreaped until you start process max+1). If your loop would finish, you can call wait_all_children() and that last process will be reaped when it finishes. That is how P::FM works. If you want different behaviour, use something else.

    Also, why is this a problem? You seem to be under the misapprehension that having one zombie process temporarily hanging around is bad. If your process let many such processes accumulate, that might become a problem. But that does not seem to be the case...so, what is the problem?

[reply]
Re: Parallel::ForkManager leaves a zombie process from last child
by vsespb (Chaplain) on May 30, 2013 at 20:54 UTC
    anyway, this seems to work better 
    while (1)
{

    $pm->start and next; 
    while ($inputcon = $daemon->accept()) {
        do_client_stuff($inputcon);
    }

    $pm->finish();
}

    [download]
    it reuses processes and I don't see zombies.
[reply]
[d/l]

      You find 1 yet-to-be reaped process wasteful, but you're ok with having N blocked processes??? I don't think you know what you want!!!

      That said, this is a better solution. You fork *before* a connection comes in, so there's less lag in handling a response. And since you reuse the child process, you don't waste time forking repeatedly. One might wonder why P::FM is used at all, but it causes children that die for whatever reason to be restarted, so you always have a full pool of children waiting!

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Re: Parallel::ForkManager leaves a zombie process from last child
by vsespb (Chaplain) on May 30, 2013 at 20:15 UTC
    I have not ever used this module. But I don't see signal handlers in its code. Probably you should handle SIGINT (Ctrl-C) by yourself. Also, related ticket https://rt.cpan.org/Public/Bug/Display.html?id=35659
[reply]
      If I kill, end the job = no problem. Problem occurs when script is working so I don't care about ctrl+c I don't want to have zombie every client connection: new client connection -> child -> do something -> end child process -> wait for another connection Last child is always a zombie. 
      5989 pts/5    S+     0:00      \_ grep test.pl
5975 pts/4    S+     0:00      \_ /usr/bin/perl ./test.pl
5987 pts/4    Z+     0:00          \_ [test.pl] <defunct>

      [download]
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[d/l]
Re: Parallel::ForkManager leaves a zombie process from last child
by RichardK (Parson) on May 30, 2013 at 18:16 UTC

    Have you called  $pm->wait_all_children;?

    It will tidy up the zombies.

    UPDATE: ignore this -- your code never ends so there's nowhere to call wait_all_children from, I guess your problem must be something else.

[reply]
[d/l]
      I can add it after 'while' but, hard to call this when script exits 'while' loop only when I kill it, Whole job is in 'while' loop.
[reply]

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