SCHED_SETAFFINITY
NAME
sched_setaffinity, sched_getaffinity, CPU_CLR, CPU_ISSET, CPU_SET, CPU_ZERO - set and get a process's CPU affinity mask
SYNOPSIS
#include <sched.h>
I int sched_setaffinity(pid_t pid , unsigned int cpusetsize ,
I cpu_set_t * mask );
I int sched_getaffinity(pid_t pid , unsigned int cpusetsize ,
I cpu_set_t * mask );
I void CPU_CLR(int cpu , cpu_set_t * set );
I int CPU_ISSET(int cpu , cpu_set_t * set );
I void CPU_SET(int cpu , cpu_set_t * set );
I void CPU_ZERO(cpu_set_t * set );
DESCRIPTION
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.
For example,
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 prevents
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
cpu_set_t
structure, a "CPU set", pointed to by
R mask .
Four macros are provided to manipulate CPU sets.
R CPU_ZERO ()
clears a set.
R CPU_SET ()
and
R CPU_CLR ()
respectively add and remove a given CPU from a set.
R CPU_ISSET ()
tests to see if a CPU is part of the set; this is useful after
R sched_getaffinity ()
returns.
The first available CPU on the system corresponds to a
cpu
value of 0, the next CPU corresponds to a
cpu
value of 1, and so on.
The constant
CPU_SETSIZE
(1024) specifies a value one greater than the maximum CPU
number that can be stored in a CPU set.
R sched_setaffinity ()
sets the CPU affinity mask of the process whose ID is
R pid
to the value specified by
R mask .
If
pid
is zero, then the calling process is used.
The argument
cpusetsize
is the length (in bytes) of the data pointed to by
R mask .
Normally this argument would be specified as
R sizeof(cpu_set_t) .
If the process specified by
pid
is not currently running on one of the CPUs specified in
R mask ,
then that process is migrated to one of the CPUs specified in
R mask .
R sched_getaffinity ()
writes the affinity mask of the process whose ID is
R pid
into the
cpu_set_t
structure pointed to by
R mask .
The
cpusetsize
argument specifies the size (in bytes) of
R mask .
If
pid
is zero, then the mask of the calling process is returned.
RETURN VALUE
On success,
R sched_setaffinity ()
and
R sched_getaffinity ()
return 0.
On error, -1 is returned, and
errno
is set appropriately.
ERRORS
EFAULT
A supplied memory address was invalid.
EINVAL
The affinity bitmask
mask
contains no processors that are physically on the system,
or
cpusetsize
is smaller than the size of the affinity mask used by the kernel.
EPERM
The calling process does not have appropriate privileges.
The process calling
R sched_setaffinity ()
needs an effective user ID equal to the user ID or effective user ID
of the process identified by
R pid ,
or it must possess the
R CAP_SYS_NICE
capability.
ESRCH
The process whose ID is pid could not be found.
VERSIONS
The CPU affinity system calls were introduced in Linux kernel 2.5.8.
The library interfaces were introduced in glibc 2.3.
Initially, the glibc interfaces included a
cpusetsize
argument.
In glibc 2.3.3, the
cpusetsize
argument was removed, but this argument was restored in glibc 2.3.4.
CONFORMING TO
These system calls are Linux specific.
NOTES
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
gettid(2)
can be passed in the argument
R pid .
A child created via
fork(2)
inherits its parent's CPU affinity mask.
The affinity mask is preserved across an
execve(2).
This manual page describes the glibc interface for the CPU affinity calls.
The actual system call interface is slightly different, with the
mask
being typed as
R unsigned long * ,
reflecting that the fact that the underlying implementation of CPU
sets is a simple bitmask.
On success, the raw
R sched_getaffinity ()
system call returns the size (in bytes) of the
cpumask_t
data type that is used internally by the kernel to
represent the CPU set bitmask.
SEE ALSO
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