I am trying to create a perl daemon that sits and runs non-stop.
I want it to create 20 child processes at a time. When one finishes, it should die and spawn another.

Here is the code I have now:

...
use constant NUM_CHILDREN => 20;
my $num_children = 0;
my $child_index = 0;
my $pm = Parallel::ForkManager->new(NUM_CHILDREN);

$pm->run_on_start( sub {
    my ($pid,$ident)=@_;
    write_file(DIR."/daemon_${ident}.pid", $pid);
    $num_children++;
    msg("$ident) Starting $pid");
});

$pm->run_on_finish( sub {
    my ($pid, $exit_code, $ident, $exit_signal, $core_dump, $job) = @_
+;
    $num_children--;
    msg("${ident}) Process completed: @_");
    if ($daemon->{'identity'}{'status'} > 0) {
         startChild($ident);
    }
});
    
# reaper subroutine
sub REAP {
    while (1) {
        my $id = waitpid( -1, WNOHANG);
        if ($id == -1) {
            return;
        }
        if ($id > 0) {
            msg("JUST REAPED $id");
        }
    }
    
    $SIG{CHLD} = \&REAP;
}

$SIG{CHLD} = \&REAP;

# start all the child processes
startChildren();

sub startChildren {
    for (1..NUM_CHILDREN) {
        startChild();
    }

    $pm->wait_all_child;
    
    msg('All children are done.');
}

sub startChild {
    my $child_id = 'child_'.++$child_index;
    my $job = shift(@{$daemon->{jobs}});
     $job->{process_id} = $child_id;
    $pm->start($child_id) and next;
    process($job);
    $pm->finish(0, $job);
}
...
[download]

Things to note:
- the work in process() IS getting completed.
- all processes are getting REAPed,
- but none are ever getting to the $pm->run_on_finish() method.
- I never get the 'All children are done.' message.
- If I do a log inside $pm->run_on_wait() i get a BUNCH of of entries.

What am I doing wrong that prevents these processes from reaching run_on_finish() ?

If things are indeed running the way they should be, then what do I need to do to make this work the way I want? What I want: start 20 child processes, have each of them die in due course, and immediately have a new one start up, via the startChild() function. Each child may be dealing with a fair amount of data, which is why I want them to die, so as to free up that memory.

I do not want to use a while(1) {} loop in the master section of the code as that tends to spike the CPU at times.

Any feedback would be most welcome.

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