simple parallel processing in perl

Question

I have a few blocks of code, inside a function of some object, that can run in parallel and speed things up for me.

I tried using subs::parallel in the following way (all of this is in a body of a function):

my $is_a_done = parallelize { 
                              # block a, do some work
                              return 1;
                             };
my $is_b_done = parallelize { 
                              # block b, do some work
                              return 1;
                             };
my $is_c_done = parallelize { 
                              # block c depends on a so let's wait (block)
                              if ($is_a_done) {
                               # do some work
                              };
                              return 1;
                             };
my $is_d_done = parallelize { 
                              # block d, do some work
                              return 1;
                             };

if ($is_a_done && $is_b_done && $is_c_done && $is_d_done) {
 # just wait for all to finish before the function returns
}

First, notice I use if to wait for threads to block and wait for previous thread to finish when it's needed (a better idea? the if is quite ugly...).

Second, I get an error:

Thread already joined at /usr/local/share/perl/5.10.1/subs/parallel.pm line 259.
Perl exited with active threads:
    1 running and unjoined
    -1 finished and unjoined
    3 running and detached

Answer 1

I haven't seen subs::parallel before, but given that it's doing all of the thread handling for you, and it seems to be doing it wrong, based on the error message, I think it's a bit suspect.

Normally I wouldn't just suggest throwing it out like that, but what you're doing really isn't any harder with the plain threads interface, so why not give that a shot, and simplify the problem a bit? At the same time, I'll give you an answer to the other part of your question.

use threads;
my @jobs;
push @jobs, threads->create(sub {
  # do some work
});

push @jobs, threads->create(sub {
  # do some other work
});

# Repeat as necessary :)

$_->join for @jobs; # Wait for everything to finish.

You need something a little bit more intricate if you're using the return values from those subs (simply switching to a hash would help a good deal) but in the code sample you provided, you're ignoring them, which makes things easy.