Author image Marty O'Brien


Sys::CpuAffinity - Set CPU affinity for processes


Version 1.12


    use Sys::CpuAffinity;

    $num_cpus = Sys::CpuAffinity::getNumCpus();

    $mask = 1 | 4 | 8 | 16;   # prefer CPU's # 0, 2, 3, 4
    $success = Sys::CpuAffinity::setAffinity($pid,$mask);
    $success = Sys::CpuAffinity::setAffinity($pid, \@preferred_cpus);

    $mask = Sys::CpuAffinity::getAffinity($pid);
    @cpus = Sys::CpuAffinity::getAffinity($pid);


The details of getting and setting process CPU affinities varies greatly from system to system. Even among the different flavors of Unix there is very little in the way of a common interface to CPU affinities. The existing tools and libraries for setting CPU affinities are not very standardized, so that a technique for setting CPU affinities on one system may not work on another system with the same architecture.

This module seeks to do one thing and do it well: manipulate CPU affinities through a common interface on as many systems as possible, by any means necessary.

The module is composed of several subroutines, each one implementing a different technique to perform a CPU affinity operation. A technique might try to import a Perl module, run an external program that might be installed on your system, or invoke some C code to access your system libraries. Usually, a technique is applicable to only a single or small group of operating systems, and on any particular system, most of the techniques would fail. Regardless of your particular system and configuration, it is hoped that at least one of the techniques will work and you will be able to get and set the CPU affinities of your processes.


No modules are required by Sys::CpuAffinity, but there are techniques for manipulating CPU affinities in other existing modules, and Sys::CpuAffinity will use these modules if they are available:

    Win32::API, Win32::Process   [MSWin32, cygwin]
    BSD::Process::Affinity       [FreeBSD]


It is important that your PATH variable is set correctly so that this module can find any external programs on your system that can help it to manipulate CPU affinities (for example, taskset on Linux, cpuset on FreeBSD).

If $ENV{DEBUG} is set to a true value, this module will produce some output that may or may not be good for debugging.


The techniques for manipulating CPU affinities for Windows (including Cygwin) and Linux have been refined and tested pretty well. Some techniques applicable to BSD systems (particularly FreeBSD) and Solaris have been tested a little bit. The hope is that this module will include more techniques for more systems in future releases. See the "NOTE TO DEVELOPERS" below for information about how you can help.

MacOS, OpenBSD are explicitly not supported, as there does not appear to be any public interface for specifying the CPU affinity of a process directly on those platforms.

On NetBSD, getting and setting CPU affinity is supported only for the calling process, and, AFAICT, only when run as the super-user. Which is to say, you can do this:

    use Sys::CpuAffinity;
    # run this process on CPUs 0, 1, 3
    Sys::CpuAffinity::setAffinity($$, [0, 1, 3]);

but not this:

    use Sys::CpuAffinity;
    $pid = `ps | grep emacs` + 0;
    # run another process on CPUs 0, 1, 3
    Sys::CpuAffinity::setAffinity($pid, [0, 1, 3]);


$bitmask = Sys::CpuAffinity::getAffinity($pid)
@preferred_cpus = Sys::CpuAffinity::getAffinity($pid)

Retrieves the current CPU affinity for the process with the specified process ID. In scalar context, returns a bit-mask of the CPUs that the process has affinity for, with the least significant bit denoting CPU #0. The return value is actually a Math::BigInt value, so it can store a bit mask on systems with an arbitrarily high number of CPUs.

In list context, returns a list of integers indicating the indices of the CPU that the process has affinity for.

So for example, if a process in an 8 core machine had affinity for cores # 2, 6, and 7, then in scalar context, getAffinity() would return

    (1 << 2) | (1 << 6) | (1 << 7) ==> 196

and in list context, it would return

    (2, 6, 7)

A return value of 0 or undef indicates an error such as an invalid process ID.

$success = Sys::CpuAffinity::setAffinity($pid, $bitmask)
$success = Sys::CpuAffinity::setAffinity($pid, \@preferred_cpus)

Sets the CPU affinity of a process to the specified processors. First argument is the process ID. The second argument is either a bitmask of the desired processors to assign to the PID, or an array reference with the index values of processors to assign to the PID.

    # two ways to assign to CPU #'s 1 and 4:
    Sys::CpuAffinity::setAffinity($pid, 0x12); # 0x12 = (1<<1) | (1<<4)
    Sys::CpuAffinity::setAffinity($pid, [1,4]);

As a special case, using a $bitmask value of -1 will clear the CPU affinities of a process -- setting the affinity to all available processors.

On some platforms, notably AIX and Irix, it is only possible to bind a process to a single CPU. If the processor mask argument to setAffinity specifies more than one processor (but less than the total number of processors in your system), then this function might only bind the process one of the specified processors.

$ncpu = Sys::CpuAffinity::getNumCpus()

Returns the module's best guess about the number of processors on this system.


This module may not work or produce undefined results on systems with more than 32 CPUs, though support for these larger systems has improved with v1.07.

Please report any bugs or feature requests to bug-sys-cpuaffinity at, or through the web interface at I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.


None known, but they are likely to arise as this module makes a lot of assumptions about how to provide input and interpret output for many different system utilities on many different platforms. Please report a bug if you suspect this module of misusing any system utilities.


You can find documentation for this module with the perldoc command.

    perldoc Sys::CpuAffinity

You can also look for information at:


This module seeks to work for as many systems in as many configurations as possible. If you know of a tool, a function, a technique to set CPU affinities on a system -- any system, -- then let's include it in this module.

Feel free to submit code through this module's request tracker:

or directly to me at <mob at> and it will be included in the next release.


BSD::Process::Affinity for demonstrating how to get/set affinities on BSD systems.

Test::Smoke::SysInfo has some fairly portable code for detecting the number of processors. provided a free OpenBSD account that allowed this module to be tested on that platform.


Marty O'Brien, <mob at>


Copyright 2010-2017 Marty O'Brien.

This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See for more information.