NAME
Parallel::Loops - Execute loops using parallel forked subprocesses
SYNOPSIS
use Parallel::Loops;
my $maxProcs = 5;
my $pl = Parallel::Loops->new($maxProcs);
my @parameters = ( 0 .. 9 );
# We want to perform some hefty calculation for each @input and
# store each calculation's result in %output. For that reason, we
# "tie" %output, so that changes to %output in any child process
# (see below) are automatically transfered and updated in the
# parent also.
my %returnValues;
$pl->share( \%returnValues );
$pl->foreach( \@parameters, sub {
# This sub "magically" executed in parallel forked child
# processes
# Lets just create a simple example, but this could be a
# massive calculation that will be parallelized, so that
# $maxProcs different processes are calculating sqrt
# simultaneously for different values of $_ on different CPUs
# (Do see 'Performance' / 'Properties of the loop body' below)
$returnValues{$_} = sqrt($_);
});
foreach (@parameters) {
printf "i: %d sqrt(i): %f\n", $_, $returnValues{$_};
}
You can also use @arrays instead of %hashes, and/or while loops instead
of foreach:
my @returnValues;
$pl->share(\@returnValues);
my $i = 0;
$pl->while ( sub { $i++ < 10 }, sub {
# This sub "magically" executed in parallel forked
# child processes
push @returnValues, [ $i, sqrt($i) ];
});
And you can have both foreach and while return values so that
$pl->share() isn't required at all:
my $maxProcs = 5;
my $pl = Parallel::Loops->new($maxProcs);
my %returnValues = $pl->foreach( [ 0..9 ], sub {
# Again, this is executed in a forked child
$_ => sqrt($_);
});
DESCRIPTION
Often a loop performs calculations where each iteration of the loop does
not depend on the previous iteration, and the iterations really could be
carried out in any order.
This module allows you to run such loops in parallel using all the CPUs
at your disposal.
Return values are automatically transfered from children to parents via
%hashes or @arrays, that have explicitly been configured for that sort
of sharing via $pl->share(). Hashes will transfer keys that are set in
children (but not cleared or unset), and elements that are pushed to
@arrays in children are pushed to the parent @array too (but note that
the order is not guaranteed to be the same as it would have been if done
all in one process, since there is no way of knowing which child would
finish first!)
If you can see past the slightly awkward syntax, you're basically
getting foreach and while loops that can run in parallel without having
to bother with fork, pipes, signals etc. This is all handled for you by
this module.
foreach loop
$pl->foreach($arrayRef, $childBodySub)
Runs $childBodySub->() with $_ set foreach element in @$arrayRef, except
that $childBodySub is run in a forked child process to obtain
parallelism. Essentially, this does something conceptually similar to:
foreach(@$arrayRef) {
$childBodySub->();
}
Any setting of hash keys or pushing to arrays that have been set with
$pl->share() will automagically appear in the hash or array in the
parent process.
If you like loop variables, you can run it like so:
$pl->foreach( \@input, sub {
my $i = $_;
.. bla, bla, bla ... $output{$i} = sqrt($i);
}
);
while loop
$pl->while($conditionSub, $childBodySub)
Essentially, this does something conceptually similar to:
while($conditionSub->()) {
$childBodySub->();
}
except that $childBodySub->() is executed in a forked child process.
Return values are transfered via share() like in "foreach loop" above.
while loops must affect condition outside $childBodySub
Note that incrementing $i in the $childBodySub like in this example will
not work:
$pl->while( sub { $i < 5 },
sub {
$output{$i} = sqrt($i);
# Won't work!
$i++
}
);
Because $childBodySub is executed in a child, and so while $i would be
incremented in the child, that change would not make it to the parent,
where $conditionSub is evaluated. The changes that make $conditionSub
return false eventually *must* take place outside the $childBodySub so
it is executed in the parent. (Adhering to the parallel principle that
one iteration may not affect any other iterations - including whether to
run them or not)
share
$pl->share(\%output, \@output, ...)
Each of the arguments to share() are instrumented, so that when a hash
key is set or array element pushed in a child, this is transfered to the
parent's hash or array automatically when a child is finished.
Note the limitation Only keys being set like "$hash{'key'} = 'value'"
and arrays elements being pushed like "push @array, 'value'" will be
transfered to the parent. Unsetting keys, or setting particluar array
elements with $array[3]='value' will be lost if done in the children.
Also, if two different children set a value for the same key, a random
one of them will be seen by the parent.
In the parent process all the %hashes and @arrays are full-fledged, and
you can use all operations. But only these mentioned operations in the
child processes make it back to the parent.
Array element sequence not defined
Note that when using share() for @returnValue arrays, the sequence of
elements in @returnValue is not guaranteed to be the same as you'd see
with a normal sequential while or foreach loop, since the calculations
are done in parallel and the children may end in an unexpected sequence.
But if you don't really care about the order of elements in the
@returnValue array then share-ing an array can be useful and fine.
If you need to be able to determine which iteration generated what
output, use a hash instead.
Recursive forking is possible
Note that no check is performed for recursive forking: If the main
process encouters a loop that it executes in parallel, and the execution
of the loop in child processes also encounters a parallel loop, these
will also be forked, and you'll essentially have $maxProcs^2 running
processes. It wouldn't be too hard to implement such a check (either
inside or outside this package).
Exception/Error Handling / Dying
If you want some measure of exception handling you can use eval in the
child like this:
my %errors;
$pl->share( \%errors );
my %returnValues = $pl->foreach( [ 0..9 ], sub {
# Again, this is executed in a forked child
eval {
die "Bogus error"
if $_ == 3;
$_ => sqrt($_);
};
if ($@) {
$errors{$_} = $@;
}
});
# Now test %errors. $errors{3} should exist as the only element
Also, be sure not to call exit() in the child. That will just exit the
child and that doesn't work. Right now, exit just makes the parent fail
no-so-nicely. Patches to this that handle exit somehow are welcome.
Performance
Properties of the loop body
Keep in mind that a child process is forked every time while or foreach
calls the provided sub. For use of Parallel::Loops to make sense, each
invocation needs to actually do some serious work for the performance
gain of parallel execution to outweigh the overhead of forking and
communicating between the processes. So while sqrt() in the example
above is simple, it will actually be slower than just running it in a
standard foreach loop because of the overhead.
Also, if each loop sub returns a massive amount of data, this needs to
be communicated back to the parent process, and again that could
outweigh parallel performance gains unless the loop body does some heavy
work too.
Linux and Windows Comparison
On the same VMware host, I ran this script in Debian Linux and Windows
XP virtual machines respectively. The script runs a "no-op" sub in 1000
child processes two in parallel at a time
my $pl = Parallel::Loops->new(2);
$pl->foreach( [1..1000], sub {} );
For comparison, that took:
7.3 seconds on Linux
43 seconds on Strawberry Perl for Windows
240 seconds on Cygwin for Windows
fork() e.g. on Windows
On some platforms the fork() is emulated. Be sure to read perlfork.
Temporary files unless select() works - e.g. on Windows
E.g. on Windows, select is only supported for sockets, and not for
pipes. So we use temporary files to store the information sent from the
child to the parent. This adds a little extra overhead. See perlport for
other platforms where there are problems with select. Parallel::Loops
tests for a working select() and uses temporary files otherwise.
SEE ALSO
This module uses fork(). ithreads could have been possible too, but was
not chosen. You may want to check out:
When to use forks, when to use threads ...?
<http://www.perlmonks.org/index.pl?node_id=709061>
The forks module (not used here)
<http://search.cpan.org/search?query=forks>
threads in perlthrtut <http://perldoc.perl.org/perlthrtut.html>
DEPENDENCIES
I believe this is the only dependency that isn't part of core perl:
use Parallel::ForkManager;
These should all be in perl's core:
use Storable;
use IO::Handle;
use Tie::Array;
use Tie::Hash;
BUGS / ENHANCEMENTS
No bugs are known at the moment. Send any reports to peter@morch.com.
Enhancements:
Optionally prevent recursive forking: If a forked child encounters a
Parallel::Loop it should be possible to prevent that Parallel::Loop
instance to also create forks.
Determine the number of CPUs so that new()'s $maxProcs parameter can be
optional. Could use e.g. Sys::Sysconf, UNIX::Processors or Sys::CPU.
Maybe use function prototypes (see Prototypes under perldoc perlsub).
Then we could do something like
pl_foreach @input {
yada($_);
};
or
pl_foreach $pl @input {
yada($_);
};
instead of
$pl->foreach(\@input, sub {
yada($_);
});
and so on, where the first suggestion above means global variables
(yikes!). Unfortunately, methods aren't supported by prototypes, so this
will never be posssible:
$pl->foreach @input {
yada($_);
};
An alternative pointed out by the perlmonks chatterbox could be to use
<Devel::Declare> "if I can stand pain".
SOURCE REPOSITORY
See the git source on github
<https://github.com/pmorch/perl-Parallel-Loops>
COPYRIGHT
Copyright (c) 2008 Peter Valdemar Mørch <peter@morch.com>
All right reserved. This program is free software; you can redistribute
it and/or modify it under the same terms as Perl itself.
AUTHOR
Peter Valdemar Mørch <peter@morch.com>