sched_setaffinity(2) System Calls Manual sched_setaffinity(2)
NAME
sched_setaffinity, sched_getaffinity - set and get a thread's CPU
affinity mask
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sched.h>
int sched_setaffinity(pid_t pid, size_t cpusetsize,
const cpu_set_t *mask);
int sched_getaffinity(pid_t pid, size_t cpusetsize,
cpu_set_t *mask);
DESCRIPTION
A thread'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 thread (i.e., setting the affinity mask
of that thread to specify a single CPU, and setting the affinity mask
of all other threads to exclude that CPU), it is possible to ensure
maximum execution speed for that thread. Restricting a thread to run
on a single CPU also avoids the performance cost caused by the cache
invalidation that occurs when a thread 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. A set of macros for manipulating CPU sets is
described in CPU_SET(3).
sched_setaffinity() sets the CPU affinity mask of the thread whose ID
is pid to the value specified by mask. If pid is zero, then the call-
ing thread 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 thread specified by pid is not currently running on one of the
CPUs specified in mask, then that thread is migrated to one of the CPUs
specified in mask.
sched_getaffinity() writes the affinity mask of the thread 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 thread is returned.
RETURN VALUE
On success, sched_setaffinity() and sched_getaffinity() return 0 (but
see "C library/kernel differences" below, which notes that the underly-
ing sched_getaffinity() differs in its return value). On failure, -1
is returned, and errno is set to indicate the error.
ERRORS
EFAULT A supplied memory address was invalid.
EINVAL The affinity bit mask mask contains no processors that are cur-
rently physically on the system and permitted to the thread ac-
cording to any restrictions that may be imposed by cpuset
cgroups or the "cpuset" mechanism described in cpuset(7).
EINVAL (sched_getaffinity() and, before Linux 2.6.9, sched_setaffin-
ity()) cpusetsize is smaller than the size of the affinity mask
used by the kernel.
EPERM (sched_setaffinity()) The calling thread does not have appropri-
ate privileges. The caller needs an effective user ID equal to
the real user ID or effective user ID of the thread identified
by pid, or it must possess the CAP_SYS_NICE capability in the
user namespace of the thread pid.
ESRCH The thread whose ID is pid could not be found.
STANDARDS
Linux.
HISTORY
Linux 2.5.8, glibc 2.3.
Initially, the glibc interfaces included a cpusetsize argument, typed
as unsigned int. In glibc 2.3.3, the cpusetsize argument was removed,
but was then restored in glibc 2.3.4, with type size_t.
NOTES
After a call to sched_setaffinity(), the set of CPUs on which the
thread will actually run is the intersection of the set specified in
the mask argument and the set of CPUs actually present on the system.
The system may further restrict the set of CPUs on which the thread
runs if the "cpuset" mechanism described in cpuset(7) is being used.
These restrictions on the actual set of CPUs on which the thread will
run are silently imposed by the kernel.
There are various ways of determining the number of CPUs available on
the system, including: inspecting the contents of /proc/cpuinfo; using
sysconf(3) to obtain the values of the _SC_NPROCESSORS_CONF and
_SC_NPROCESSORS_ONLN parameters; and inspecting the list of CPU direc-
tories under /sys/devices/system/cpu/.
sched(7) has a description of the Linux scheduling scheme.
The affinity mask is a per-thread attribute that can be adjusted inde-
pendently for each of the threads in a thread group. The value re-
turned from a call to gettid(2) can be passed in the argument pid.
Specifying pid as 0 will set the attribute for the calling thread, and
passing the value returned from a call to getpid(2) will set the at-
tribute for the main thread of the thread group. (If you are using the
POSIX threads API, then use pthread_setaffinity_np(3) instead of
sched_setaffinity().)
The isolcpus boot option can be used to isolate one or more CPUs at
boot time, so that no processes are scheduled onto those CPUs. Follow-
ing the use of this boot option, the only way to schedule processes
onto the isolated CPUs is via sched_setaffinity() or the cpuset(7)
mechanism. For further information, see the kernel source file Docu-
mentation/admin-guide/kernel-parameters.txt. As noted in that file,
isolcpus is the preferred mechanism of isolating CPUs (versus the al-
ternative of manually setting the CPU affinity of all processes on the
system).
A child created via fork(2) inherits its parent's CPU affinity mask.
The affinity mask is preserved across an execve(2).
C library/kernel differences
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 the fact that the
underlying implementation of CPU sets is a simple bit mask.
On success, the raw sched_getaffinity() system call returns the number
of bytes placed copied into the mask buffer; this will be the minimum
of cpusetsize and the size (in bytes) of the cpumask_t data type that
is used internally by the kernel to represent the CPU set bit mask.
Handling systems with large CPU affinity masks
The underlying system calls (which represent CPU masks as bit masks of
type unsigned long *) impose no restriction on the size of the CPU
mask. However, the cpu_set_t data type used by glibc has a fixed size
of 128 bytes, meaning that the maximum CPU number that can be repre-
sented is 1023. If the kernel CPU affinity mask is larger than 1024,
then calls of the form:
sched_getaffinity(pid, sizeof(cpu_set_t), &mask);
fail with the error EINVAL, the error produced by the underlying system
call for the case where the mask size specified in cpusetsize is
smaller than the size of the affinity mask used by the kernel. (De-
pending on the system CPU topology, the kernel affinity mask can be
substantially larger than the number of active CPUs in the system.)
When working on systems with large kernel CPU affinity masks, one must
dynamically allocate the mask argument (see CPU_ALLOC(3)). Currently,
the only way to do this is by probing for the size of the required mask
using sched_getaffinity() calls with increasing mask sizes (until the
call does not fail with the error EINVAL).
Be aware that CPU_ALLOC(3) may allocate a slightly larger CPU set than
requested (because CPU sets are implemented as bit masks allocated in
units of sizeof(long)). Consequently, sched_getaffinity() can set bits
beyond the requested allocation size, because the kernel sees a few ad-
ditional bits. Therefore, the caller should iterate over the bits in
the returned set, counting those which are set, and stop upon reaching
the value returned by CPU_COUNT(3) (rather than iterating over the num-
ber of bits requested to be allocated).
EXAMPLES
The program below creates a child process. The parent and child then
each assign themselves to a specified CPU and execute identical loops
that consume some CPU time. Before terminating, the parent waits for
the child to complete. The program takes three command-line arguments:
the CPU number for the parent, the CPU number for the child, and the
number of loop iterations that both processes should perform.
As the sample runs below demonstrate, the amount of real and CPU time
consumed when running the program will depend on intra-core caching ef-
fects and whether the processes are using the same CPU.
We first employ lscpu(1) to determine that this (x86) system has two
cores, each with two CPUs:
$ lscpu | egrep -i 'core.*:|socket'
Thread(s) per core: 2
Core(s) per socket: 2
Socket(s): 1
We then time the operation of the example program for three cases: both
processes running on the same CPU; both processes running on different
CPUs on the same core; and both processes running on different CPUs on
different cores.
$ time -p ./a.out 0 0 100000000
real 14.75
user 3.02
sys 11.73
$ time -p ./a.out 0 1 100000000
real 11.52
user 3.98
sys 19.06
$ time -p ./a.out 0 3 100000000
real 7.89
user 3.29
sys 12.07
Program source
#define _GNU_SOURCE
#include <err.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
int parentCPU, childCPU;
cpu_set_t set;
unsigned int nloops;
if (argc != 4) {
fprintf(stderr, "Usage: %s parent-cpu child-cpu num-loops\n",
argv[0]);
exit(EXIT_FAILURE);
}
parentCPU = atoi(argv[1]);
childCPU = atoi(argv[2]);
nloops = atoi(argv[3]);
CPU_ZERO(&set);
switch (fork()) {
case -1: /* Error */
err(EXIT_FAILURE, "fork");
case 0: /* Child */
CPU_SET(childCPU, &set);
if (sched_setaffinity(getpid(), sizeof(set), &set) == -1)
err(EXIT_FAILURE, "sched_setaffinity");
for (unsigned int j = 0; j < nloops; j++)
getppid();
exit(EXIT_SUCCESS);
default: /* Parent */
CPU_SET(parentCPU, &set);
if (sched_setaffinity(getpid(), sizeof(set), &set) == -1)
err(EXIT_FAILURE, "sched_setaffinity");
for (unsigned int j = 0; j < nloops; j++)
getppid();
wait(NULL); /* Wait for child to terminate */
exit(EXIT_SUCCESS);
}
}
SEE ALSO
lscpu(1), nproc(1), taskset(1), clone(2), getcpu(2), getpriority(2),
gettid(2), nice(2), sched_get_priority_max(2),
sched_get_priority_min(2), sched_getscheduler(2),
sched_setscheduler(2), setpriority(2), CPU_SET(3), get_nprocs(3),
pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7), cpuset(7),
sched(7), numactl(8)
Linux man-pages 6.7 2023-10-31 sched_setaffinity(2)
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