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SCHED_SETAFFINITY(2) Linux Programmer's Manual SCHED_SETAFFINITY(2)
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 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 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 num-
ber 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 calling process is used.
The argument cpusetsize is the length (in bytes) of the data pointed to by mask.
Normally this argument would be specified as sizeof(cpu_set_t).
If the process specified by pid is not currently running on one of the CPUs speci-
fied 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.
RETURN VALUE
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 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 call-
ing sched_setaffinity() needs an effective user ID equal to the user ID or
effective user ID of the process identified by pid, or it must possess the
CAP_SYS_NICE capability.
ESRCH The process whose ID is pid could not be found.
CONFORMING TO
These system calls are Linux specific.
NOTES
The affinity mask is actually a per-thread attribute that can be adjusted indepen-
dently 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 success, 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.
HISTORY
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 2.3.2, the cpusetsize argument was removed, but
this argument was restored in glibc 2.3.4.
SEE ALSO
clone(2), getpriority(2), gettid(2), nice(2), sched_get_priority_max(2),
sched_get_priority_min(2), sched_getscheduler(2), sched_setscheduler(2), setprior-
ity(2), capabilities(7)
sched_setscheduler(2) has a description of the Linux scheduling scheme.
Linux 2006-02-03 SCHED_SETAFFINITY(2)
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