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Parallel::Simple - the simplest way to run code blocks in parallel


 use Parallel::Simple qw( prun );

 # style 1: simple list of code blocks
     sub { print "$$ foo\n" },
 ) or die( Parallel::Simple::errplus() );

 # style 1 with options
     sub { print "$$ foo\n" },
     { use_return => 1 },
 ) or die( Parallel::Simple::errplus() );

 # style 2: named code blocks (like the Benchmark module)
     foo => sub { print "$$ foo\n" },
     bar => \&my_bar_func,
 ) or die( Parallel::Simple::errplus() );

 # getting fancy with arg binding
     [ \&my_bar_func, @args ],
     [ \&my_bar_func, @other_args ],
 ) or die( Parallel::Simple::errplus() );


I generally write my scripts in a linear fashion. Do A, then B, then C. However, I often have parts that don't depend on each other, and therefore don't have to run in any particular order, or even linearly. I could save time by running them in parallel - but I'm too lazy to deal with forking, and reaping zombie processes, and other nastiness.

The goal of this module is to make it so mind-numbingly simple to run blocks of code in parallel that there is no longer any excuse not to do it, and in the process, drastically cut down the runtimes of many of our applications, especially when running on multi-processor servers (which are pretty darn common these days).

Parallel code execution is now as simple as calling prun and passing it a list of code blocks to run, followed by testing the return value for truth using the common "or die" perl idiom.


By default, Parallel::Simple does not export any symbols. You generally want to export the prun function to save time:

    use Parallel::Simple qw(prun);
    prun( ... );

No other functions are exportable. You will have to full qualify calls to them, like this:

    print Parallel::Simple::errplus();



Runs multiple code blocks in parallel by forking a process for each one and returns when all processes have exited. Returns true if nothing went wrong, false otherwise. Therefore, returns true if no code blocks are passed, since nothing can go wrong if there are no code blocks to execute!

There are two styles of passing code blocks:

Style 1 - Simple List of Code Blocks

In its simplest form (which is what we're all about here), prun takes a list of code blocks and then forks a process to run each block. It returns true if all processes exited with exit value 0, false otherwise. Example:

        sub { print "$$ foo\n" },
    ) or die( Parallel::Simple::errplus() );

By default, the exit value will be 255 if the code block dies or throws any exceptions, or 0 if it doesn't. You can exercise more control over this by using the use_return option (documented below) and returning values from your code block.

If prun returns false and you want to see what went wrong, try the err, errplus, and rv functions documented below - especially rv which will tell you the exit values of the processes that ran the code blocks.

Style 2 - Named Code Blocks

Alternatively, you can specify names for all of your code blocks by using the common "named params" perl idiom. The only benefit you get from this currently is an improved lookup method for code block return values (see rv for more details). Example:

        foo => sub { print "$$ foo\n" },
        bar => \&my_bar_func,
    ) or die( Parallel::Simple::errplus() );

Other than looking nicer, this behaves identically to the Style 1 example.

Argument Binding

You can include arguments to be passed to the code block along with the block itself by passing a listref containing the code block followed by the args:

        [ \&my_bar_func, @args ],
        [ \&my_bar_func, @other_args ],
    ) or die( Parallel::Simple::errplus() );

This will fork two processes, one of which will call &my_bar_func and pass it @args, the other of which will call &my_bar_func and pass it @other_args. This works with both styles. Here a similar example using named code blocks:

        foo  => sub { print "$$ foo\n" },
        bar1 => [ \&my_bar_func, @args ],
        bar2 => [ \&my_bar_func, @other_args ],
    ) or die( Parallel::Simple::errplus() );


You can optionally pass a reference to a hash containing additional options as the last argument. Example:

        sub { print "$$ foo\n" },
        { use_return => 1 },
    ) or die( Parallel::Simple::errplus() );

There are currently only two options:


By default, the return values for the code blocks, which are retrieved using the rv function, will be 0 if the code block executed normally or 255 if the code block died or threw any exceptions. By default, any value the code block returns is ignored.

If you use the use_return option, then the return value of the code block is used as the return value (unless the code block dies or throws an exception, in which case the return value will still be 255). This value is passed to the exit function, so please please please use only number between 0 and 255!

Note: If you use the abort_on_error option, blocks may also have a return value of -1 indicating that they were terminated because another block failed.


Normally, a call to prun will block until all code blocks have exited, regardless of exit status. If you use the abort_on_error option, all blocks will be terminated immediately as soon as any block exits unsuccessfully. Note that terminated blocks will have a return value of -1.


Returns a string describing the last error that occured, or undef if there has not yet been any errors.

Currently, only two error messages are possible:

  • if the call to fork fails, err returns the contents of $!

  • if any blocks fail, err returns a message describing how many blocks failed out of the total


Returns a string containing the return value of err plus a nicely formatted version of the return value of rv. I strongly recommend using this function in the following idiom, because it is the laziest way to present the greatest amount of failure/debugging information to the user:

      ...  # code blocks
    ) or die( Parallel::Simple::errplus() );

Returns different value types depending on whether or not you used named code blocks:

Style 1 (not using named code blocks)

returns a reference to an array containing the return values of the code blocks in the order they were passed to prun

Style 2 (using named code blocks)

returns a reference to a hash, where keys are the code block names, and values are the return values of the respective code block

See the use_return option for the prun function for more details on how to control return values.


This module was developed and tested on Red Hat Linx 9.0, kernel 2.6.11, and perl v5.8.4 built for i686-linux-thread-multi. I have not tested it anywhere else. This module is obviously limited to platforms that have a working fork implementation.

I would appreciate any feedback from people using this module in different environments, whether it worked or not, so I can note it here.


The world could probably use a thread-based version of the prun function. I'm currently accepting applications from volunteer coders. :)


 - capture stdout/stderr
 - capture block die/exception messages
 - capture entire 16 bit exit code, just in case

However, these all make Parallel::Simple less Simple.


Parallel::ForkControl, Parallel::ForkManager, and Parallel::Jobs are all similarly themed, and offer different interfaces and advanced features. I suggest you skim the docs on all three (in addition to mine) before choosing the right one for you. Or you can foolishly trust the executive summaries below:


Only takes one subroutine reference to run, but provides wonderful ways to control how many children are forked and keeping activity below certain thresholds. Arguments to the run method will be passed on to the subroutine you specified during construction, so there's some run-time flexibility. It is not yet possible to learn anything about what happened to the forked children, such as inspecting return or exit values.

Conclusion: Best for repetitive, looped tasks, such as fetching web pages or running a command across a cluster of machines in parallel.

Incidentally, Parallel::ForkControl would be far more useful with the following two changes:

  • Support some kind of feedback - return/exit values at a minimum, or even a single value summary, like the return value of my prun function.

  • Allow the user to specify the Code value in the run method instead of during construction. Then Parallel::ForkControl could do everything this module does and more, albeit with a more sophisticated interface.


Unique in the Parallel::* world in that it keeps the user somewhat involved in the forking process. Rather than taking a code reference, you call the start() method to fork and test the return value to determine whether you are now the parent or child... almost like just calling fork yourself. :)

Provides control over how many child processes to allow, and blocks new forks until some previous children have exited. Let's child determine the process exit value. Provides a trigger mechanism to run callbacks when certain events happen (child start, child exit, and start blocking). You must supply a callback for the child exit event to inspect the exit value of the child.

Conclusion: While also designed for repetitive, looped tasks, it is far more flexible, being a thin wrapper around fork rather than taking over child creation and management entirely. Useful mostly if you want to limit child processes to a certain number at a time and/or if the native system calls scare you.


Different in that it executes shell commands as opposed to subroutines or code blocks. Provides all the features of the open3 function, including explicit control over STDIN, STDOUT, and STDERR on all 'jobs'. Lets you monitor jobs for output and exit events (with associated details for each event).

Conclusion: Great for shell commands! Not great for not shell commands.


Written by Ofer Nave <>. Sponsered by Shopzilla, Inc. (formerly


Copyright 2005 by Shopzilla, Inc.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.