parallel functions

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

Hi Monks,

I'm trying to run two functions in parallel thusly:

#!/usr/bin/perl
use 5.014;
my @child_pids;

for my $cmd (\&test1, \&test2) 
{
    defined(my $child_pid = fork()) or die "Couldn't fork: $!";
    if ($child_pid == 0) 
    {
       exec $cmd->();
    }   
    else 
    {
       push @child_pids, $child_pid;
    }
}

for my $pid (@child_pids) {
   waitpid($pid, 0);
}

sub test1
{
sleep 3;
say "in test1";
}

sub test2
{
sleep 2;  
say "in test2";
}
[download]

Output:

in test2
in test1
in test2
[download]

Say what? Why is it running test2 twice?
thanks for any responses

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Replies are listed 'Best First'.
Re: parallel functions by kcott (Archbishop) on Nov 07, 2012 at 21:58 UTC
G'day micmac, If you add `use warnings;`, you'll see that the exec is not being run at all: `$ pm_child_execs.pl in test2 Can't exec "1": No such file or directory at ./pm_child_execs.pl line +13. in test1 Can't exec "1": No such file or directory at ./pm_child_execs.pl line +13. in test2 Can't exec "1": No such file or directory at ./pm_child_execs.pl line +13.` [download] What's actually happening is that `$cmd->()` is being run then exec is trying to use the return value (`"1"` from say) as its argument. So, the first child runs `test1()` then, because there's no exec, it runs `test2()` on the second iteration. The second child runs `test2()` (also with no exec). That's `test1()` once and `test2()` twice. Changing `exec $cmd->();` [download] to `#exec $cmd->(); $cmd->(); last;` [download] you'll get the desired output: `$ pm_child_execs.pl in test2 in test1` [download] I'll leave you to decide if that's also the desired functionality. -- Ken	[reply] [d/l] [select]
Re^2: parallel functions by micmac (Acolyte) on Nov 07, 2012 at 22:11 UTC
Good explanation, thanks Ken.	[reply]
Re: parallel functions by tobyink (Canon) on Nov 07, 2012 at 21:40 UTC
Because the first fork creates two processes. One of those processes calls test1, the other does not. Then both processes fork again, creating a total of four processes; two of those processed call test2. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply]
Re: parallel functions by chromatic (Archbishop) on Nov 07, 2012 at 21:41 UTC
Did you mean exit instead of exec? Hint: what's your forked child going to do when then function reference returns? Improve your skills with Modern Perl: the free book.	[reply]
Re: parallel functions by marioroy (Prior) on Nov 25, 2012 at 06:26 UTC
A new parallel module was recently added to CPAN called MCE. This is how one could run 2 functions in parallel using MCE. `#!/usr/bin/perl use 5.014; use MCE; my $mce = MCE->new( max_workers => 2, user_func => sub { my $self = shift; if ($self->wid == 1) { test1(); } else { test2(); } } ); $mce->run(); sub test1 { sleep 3; say "in test1"; } sub test2 { sleep 2; say "in test2"; }` [download] Output `in test2 in test1` [download]	[reply] [d/l] [select]
Re: parallel functions by Anonymous Monk on Nov 07, 2012 at 21:53 UTC
I did indeed mean exit. Thanks for both responses.	[reply]