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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>
int sched_setaffinity(pid_t pid, unsigned int cpusetsize,
cpu_set_t *mask);
int sched_getaffinity(pid_t pid, unsigned int cpusetsize,
cpu_set_t *mask);
void CPU_CLR(int cpu, cpu_set_t *set);
int CPU_ISSET(int cpu, cpu_set_t *set);
void CPU_SET(int cpu, cpu_set_t *set);
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 dedi‐
cating 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 mask. Four macros are provided to manipulate CPU
sets. CPU_ZERO() clears a set. CPU_SET() and CPU_CLR() respectively
add and remove a given CPU from a set. CPU_ISSET() tests to see if a
CPU is part of the set; this is useful after 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.
sched_setaffinity() sets the CPU affinity mask of the process whose ID
is pid to the value specified by mask. If pid is zero, then the call‐
ing process is used. The argument cpusetsize is the length (in bytes)
of the data pointed to by mask. Normally this argument would be speci‐
fied as sizeof(cpu_set_t).
If the process specified by pid is not currently running on one of the
CPUs specified in mask, then that process is migrated to one of the
CPUs specified in mask.
sched_getaffinity() writes the affinity mask of the process whose ID is
pid into the cpu_set_t structure pointed to by mask. The cpusetsize
argument specifies the size (in bytes) of mask. If pid is zero, then
the mask of the calling process is returned.
On success, sched_setaffinity() and 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 physi‐
cally 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 sched_setaffinity() needs an effective user ID
equal to the user ID or effective user ID of the process identi‐
fied by pid, or it must possess the 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.
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
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 unsigned long *, reflecting that the fact that
the underlying implementation of CPU sets is a simple bitmask. On suc‐
cess, the raw 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.
clone(2), getpriority(2), gettid(2), nice(2), sched_get_prior
ity_max(2), sched_get_priority_min(2), sched_getscheduler(2),
sched_setscheduler(2), setpriority(2), capabilities(7)
sched_setscheduler(2) has a description of the Linux scheduling scheme.