A process's CPU affinity mask determines the set of CPUs on which
it is eligible to run.
On a multiprocessor system, setting the CPU affinity mask
can be used to obtain performance benefits.
by dedicating one CPU to a particular process
(i.e., setting the affinity mask of that process to specify a single CPU,
and setting the affinity mask of all other processes to exclude that CPU),
it is possible to ensure maximum execution speed for that process.
Restricting a process to run on a single CPU also avoids
the performance cost caused by the cache invalidation that occurs
when a process ceases to execute on one CPU and then
recommences execution on a different CPU.
A CPU affinity mask is represented by the
structure, a "CPU set", pointed to by
A set of macros for manipulating CPU sets is described in
sets the CPU affinity mask of the process whose ID is
to the value specified by
is zero, then the calling process is used.
is the length (in bytes) of the data pointed to by
Normally this argument would be specified as
If the process specified by
is not currently running on one of the CPUs specified in
then that process is migrated to one of the CPUs specified in
writes the affinity mask of the process whose ID is
structure pointed to by
argument specifies the size (in bytes) of
is zero, then the mask of the calling process is returned.
On error, -1 is returned, and
is set appropriately.
A supplied memory address was invalid.
The affinity bit mask
contains no processors that are currently physically on the system
and permitted to the process according to any restrictions that
may be imposed by the "cpuset" mechanism described in
and, in kernels before 2.6.9,
is smaller than the size of the affinity mask used by the kernel.
The calling process does not have appropriate privileges.
The caller needs an effective user ID equal to the user ID
or effective user ID of the process identified by
or it must possess the
The process whose ID is pid could not be found.
The CPU affinity system calls were introduced in Linux kernel 2.5.8.
The system call wrappers were introduced in glibc 2.3.
Initially, the glibc interfaces included a
argument, typed as
In glibc 2.3.3, the
argument was removed, but was then restored in glibc 2.3.4, with type
These system calls are Linux-specific.
After a call to
the set of CPUs on which the process will actually run is
the intersection of the set specified in the
argument and the set of CPUs actually present on the system.
The system may further restrict the set of CPUs on which the process
runs if the "cpuset" mechanism described in
is being used.
These restrictions on the actual set of CPUs on which the process
will run are silently imposed by the kernel.
The affinity mask is actually a per-thread attribute that can be
adjusted independently for each of the threads in a thread group.
The value returned from a call to
can be passed in the argument
as 0 will set the attribute for the calling thread,
and passing the value returned from a call to
will set the attribute for the main thread of the thread group.
(If you are using the POSIX threads API, then use
A child created via
inherits its parent's CPU affinity mask.
The affinity mask is preserved across an
This manual page describes the glibc interface for the CPU affinity calls.
The actual system call interface is slightly different, with the
being typed as
unsigned long *,
reflecting the fact that the underlying implementation of CPU
sets is a simple bit mask.
On success, the raw
system call returns the size (in bytes) of the
data type that is used internally by the kernel to
represent the CPU set bit mask.