++ed by:
Author image Josh Rabinowitz


Proc::NiceSleep - yield system in an intelligent fashion


  use Proc::NiceSleep qw( :all ); 
  nice(5);              # lower our priority, if our OS supports it 
  max_load(1.1);        # max load we allow, if GetCpuLoad can find loads
  sleep_factor(.5);     # sleep 50% as long as we run
  min_run_time(2);      # run at least 2 seconds without sleep
  while($somecondition) {
    $slept = maybe_sleep(); # sleep some amount of time if needed 


Proc::NiceSleep defines subroutines to allow a process to yield use of the system according to a configured policy.

Proc::NiceSleep is intended for use in situations where the operating system does not support priorities, or where using the operating system's built-in priorities does not yield the system sufficiently.

By default Proc::NiceSleep expects to yield the process for one tenth the amount of time that process runs. This is expressed by the default Sleep Factor of 0.10 and can be set via sleep_factor( $n ).

Proc::NiceSleep can also be configured to attempt to keep the average system load below a certain threshhold through use of the max_load() function.

A convenient nice() function, which acts much like the shell command and executable of the same name, is also provided for easy, platform independent access to your system's priorities (if available).

If Proc::NiceSleep autodetects the presence of the Time::HiRes module (and your operating system supports it) then timing and yielding operations will occur with sub-second granularity. If not, no warning or error will be issued but Proc::NiceSleep operations will occur with a granularity of about one second. Sys::CpuLoad must be found for max_load() to have any effect.

The following functions can be imported from this module.

maybe_sleep ()

Checks to see if this process should yield use of the system by issuing some kind of sleep at this point, and if so, does so for an appropriate amount of time. Returns 0 if no sleep was performed, otherwise returns the amount of seconds maybe_sleep() actually slept for.

maybe_sleep ( $maxsleeptime )

Calls maybe_sleep() until it returns 0 or $maxsleeptime has passed. Returns the sum of the times maybe_sleep() slept.

max_load ( $max_load )

Set or gets the maximum 1-minute average load allowed to occur before a sleep call will be issued by maybe_sleep(), depending on whether a parameter is passed or not. The default value of 0 disables this feature; setting the maximum load will only have an effect if Sys::CpuLoad is successfully loaded (or an alternate load retrieval function is provided through load_function()). This module will check the system load no more than about once per second. If both sleep_factor() and max_load() are used then maybe_sleep() will yield the system if either condition is met.

sleep_factor ( $factor )

Sets or gets the sleep factor depending on whether a number is passed or not. A sleep factor of 1 means to sleep an equal amount of time as we run, 2 means to sleep twice as long, and so on. The default value is 0.1. If the sleep factor is set to zero, then this feature is disabled. If both sleep_factor() and max_load() are used then maybe_sleep() will yield the system if either condition is met.

nice ()

Sets or gets the priority of the process, as understood by the operating system. If passed an integer, nice() attempts to set priority of the process to the value specified, and returns that value. If no parameter is passed, nice() attempts to query the operating system for the priority of the process and return it. If your OS doesn't support priorities then nice() will likely have no effect and always return 0.

The exact nice() values returned and recognized, and their meanings to the system, are system dependent but usually range from about -20 (highest priority) to 20 (lowest priority, 'nicest').

min_run_time ()

Sets or gets the minimum run time, in seconds, depending on whether a number is passed or not. The minumum run time is the least amount of time that Proc::NiceSleep will allow the process to run between sleeps. The default value is 0 seconds.

min_sleep_time ()

Sets or gets the minimum amount time, in seconds, that maybe_sleep() will sleep for if it detects that a sleep is appropriate. The default it 0.

over_load_min_sleep_time ()

Sets or gets the minimum amount time, in seconds, that maybe_sleep() will sleep when the load has gone above the max_load(). The default is 3.5 seconds.

over_load_sleep_drift ()

Sets or gets the 'drift' in the amount time, that maybe_sleep() will sleep when the load has gone above the value set with max_load(). The actual amount of time chosen to sleep will be between over_load_min_sleep_time() and over_load_min_sleep_time() + ( 4 * (curload - targetload) * over_load_sleep_drift() ) seconds (that is, it will sleep longer as the system load goes up.) The default is over_load_sleep_drift is 1.

load_function( \&function )

Sets or gets a reference to a function to be used by maybe_sleep() that returns numbers that should be considered the 1, 5, and 15-minute average system loads. (Only the 1-minute average is used). Set to undef (the default) to use your system's load.

reset_all ()

Resets the internal statistics to act as if the process had just started.

DumpText ()

Returns a string (intended for display) containing multiple lines with internal information about Proc::NiceSleep's runtime configuration and statistics. The format and contents of the returned string are intended for informational and debugging use and are subject to change.

Dump ()

Returns a reference to a hash with internal information about Proc::NiceSleep configuration and statistics. The names and presence of the returned hash names and values are for informational and debugging purposes only and are subject to change. Modifying the returned hash will have no effect on the operation of Proc::NiceSleep.


None by default.


Josh Rabinowitz, <Josh Rabinowitz>


The meanings of values accepted by nice() may vary between operating systems (e.g. HP-UX). This problem is to be addressed in future revisions to this package; for now be advised that use of nice() is not necessarily portable.

Uncoordinated use of sleep() (and possibly of signal() and alarm()) in your perl program may cause your program to yield the system more or less than specified via Proc::NiceSleep policies.


Time::HiRes, Sys::CpuLoad


Copyright (c) 2002-2012 Josh Rabinowitz. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


Proc::NiceSleep is loosely modeled on Lincoln Stein's CGI.pm, and on D. Wegscheid and other's Time::HiRes.pm. Thanks to Michael G Schwern, Terrence Brannon, and David Alban for their valuable input.


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