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Jason McCarver
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Parallel::ForkManager::Scaled - Run processes in parallel based on CPU usage


Version 0.17


    use Parallel::ForkManager::Scaled;

    # my $pm = Parallel::ForkManager::Scaled->new( attrib => value, ... );
    my $pm = Parallel::ForkManager::Scaled->new;

    # Used just like Parallel::ForkManager, so I'll paraphrase its documentation

    for my $data (@all_data) {
        # $pid is set to the child process' PID
        my $pid = $pm->start and next;

        # In the child process now
        # do some work ..

        # Exit the child


This module inherits from Parallel::ForkManager and adds the ability to automatically manage the number of processes running based on how busy the system is by watching the CPU idle time. Each time a child is about to be start()ed a new value for max_procs may be calculated (if enough time has passed since the last calculation). If a new value is calculated, the number of processes to run will be adjusted by calling set_max_procs with the new value.

Without specifying any attributes to the constructor, some defaults will be set for you (see Attributes below).


Attributes are just methods that may be passed to the constructor (new()) and most may be changed during the life of the returned object. They take as a parameter a new value to set for the attribute and return the current value (or new value if one was passed).


The number of running processes will never be adjusted lower than this value.

default: 1


The number of running processes will never be adjusted higher than this value.

default: The detected number of CPUs * 2


This is initially set to hard_min_procs and hard_max_procs respectively and is adjusted over time. These are used when calculating adjustments as the minimum and maximum number of processes respectively.

Over time soft_min_procs and soft_max_procs should approach the same value for a consistent workload and a machine not otherwise busy.

Depending on the needs of the system, these values may also diverge if necessary to try to reach idle_target.

You may adjust these values if you wish by passing a value to the method but you probably shouldn't. :)

initial_procs (read-only)

The number of processes to start running before attempting any adjustments, max_procs will be set to this value upon initialization.

default: half way between soft_min_procs and soft_max_procs


The minimum amount of time, in seconds, that must elapse between checks of the system CPU's idle % and updates to the number of running processes.

Set this to 0 to cause a check before each call to start().

Before each call to start() the time is compared with the last time a check/update was performed. If this much time has passed, a new check will be made of how busy the CPU is and the number of processes may be adjusted.

default: 1


Percentage of CPU idle time to try to maintain by adjusting the number of running processes between hard_min_procs and hard_max_procs

default: 0 # try to keep the CPU 100% busy (0% idle)


Only make adjustments if the current CPU idle % is this distance away from idle_target. In other words, only adjust if abs(cur_idle - idle_target) > idle_threshold. This may be a fractional value (floating point).

You may notce that the default idle_target of 0 and idle_threshold of 1 would seem to indicate that the processes would never be adjusted as idle can never be less than 0%. At the limits, the threshold is adjusted so that we will still attempt adjustments, something like this:

    min_ok = max(0,   idle_target - idle_threshold)
    max_ok = min(100, idle_target - idle_threshold)

    adjust if idle >= max_ok
    adjust if idle <= min_ok

default: 1


This is a callback function that is run immediately after (possibly) adjusting max_procs. This allows you to override the default behavior of this module for your own nefarious purposes.

run_on_update expects a coderef which will be called with two parameters:

  • The object being adjusted. ($obj)

  • The old value for $obj->max_procs. If you decide you have a better idea of what max_procs should be, in your callback just set it via $obj->set_max_procs($new_value).

The return value from the callback is ignored.


  $pm->run_on_update( sub{
      my ($obj, $old_max_procs) = @_;

This is passed to the Parallel::ForkManager constructor to set tempdir. Where Parallel::ForkManager is constructed thusly:

  my $pm = Parallel::ForkManager->new($procs, $tempdir);

The equivalent for this module would be:

  my $pm = Parallel::ForkManager::Scaled->new(initial_procs => $procs, tempdir => $tempdir);


All methods inherited from Parallel::ForkManager plus the following:


Print the string returned by stats to STDERR. This may be used as a callback with run_on_update to see diagnostics as processes are run:


Returns the last time() stats were updated via update_stats_pct.


Returns the system's idle percentage as of last_update.

Note that it's possible for idle to be NaN if not enough time has elapsed between the when the object was built and the most recent call to update_stats_pct. Once enough time has elapsed for an idle % to be calculated, idle will never contain an NaN value.


The number of CPUs detected on the system, this is just a wrapper to the cpus function from Unix::Statgrab.


This method overrides set_max_procs from Parallel::ForkManager and automatically constrains the new value to be within soft_min_procs and soft_max_procs inclusive.


Returns a formatted string with information about the current status. Takes a single parameter, the old value for max_procs. If no parameter is passed, the vlaue max_procs will be used.

Method(s) you probably don't need to use

These are not meant for general consumption but are available anyway. Probably best to avoid them :)


This method will attempt to update CPU stats (idle, etc). It is automatically called before each child process is start()ed if at least update_frequency seconds has elapsed since the last call.

If not enough time has elapsed since the last call to update_stats_pct it's possible to get NaN for the new idle stat. In this case no updates will be made.

If idle is updated, last_update will also be updated with the time.


These examples are also provided in the examples/ directory of this distribution.

Maximize CPU usage

see: examples/prun.pl

Run shell commands that are passed into the program and try to keep the CPU busy, i.e. 0% idle

    use Parallel::ForkManager::Scaled;

    my $pm = Parallel::ForkManager::Scaled->new(
        run_on_update => \&Parallel::ForkManager::Scaled::dump_stats
    # just to be sure we can saturate the CPU
    $pm->hard_max_procs($pm->ncpus * 4);


    while (<>) {
        $pm->start and next;

        # In the child now, run the shell process
        system $_;

Dummy Load

see: examples/dummy_load.pl

This example provides a way to test the capabilities of this module. Try changing the idle_target and other settings to see the effect.

    use Parallel::ForkManager::Scaled;

    my $pm = Parallel::ForkManager::Scaled->new(
        run_on_update => \&Parallel::ForkManager::Scaled::dump_stats,
        idle_target => 50,


    for my $i (0..1000) {
        $pm->start and next;

        my $start = time;
        my $lifespan = 5+int(rand(10));

        # Keep the CPU busy until it's time to exit
        while (time - $start < $lifespan) { 
            my $a = time; 
            my $b = $a^time/3;



Currently this module only works on systems where Unix::Statgrab is available, which is probably any system where the libstatgrab library can compile.


Jason McCarver <slam@parasite.cc>




The mercurial repository for this module may be found here:


You can clone it with

  hg clone https://bitbucket.org/jmccarv/parallel-forkmanager-scaled


This software is copyright (c) 2016 by Jason McCarver

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.