1SCHED_SETAFFINITY(2)       Linux Programmer's Manual      SCHED_SETAFFINITY(2)
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3
4

NAME

6       sched_setaffinity,  sched_getaffinity  -  set  and  get  a thread's CPU
7       affinity mask
8

SYNOPSIS

10       #define _GNU_SOURCE             /* See feature_test_macros(7) */
11       #include <sched.h>
12
13       int sched_setaffinity(pid_t pid, size_t cpusetsize,
14                             const cpu_set_t *mask);
15
16       int sched_getaffinity(pid_t pid, size_t cpusetsize,
17                             cpu_set_t *mask);
18

DESCRIPTION

20       A thread's CPU affinity mask determines the set of CPUs on which it  is
21       eligible  to run.  On a multiprocessor system, setting the CPU affinity
22       mask can be used to obtain performance benefits.  For example, by dedi‐
23       cating  one CPU to a particular thread (i.e., setting the affinity mask
24       of that thread to specify a single CPU, and setting the  affinity  mask
25       of  all  other  threads  to exclude that CPU), it is possible to ensure
26       maximum execution speed for that thread.  Restricting a thread  to  run
27       on  a  single  CPU also avoids the performance cost caused by the cache
28       invalidation that occurs when a thread ceases to execute on one CPU and
29       then recommences execution on a different CPU.
30
31       A  CPU  affinity mask is represented by the cpu_set_t structure, a "CPU
32       set", pointed to by mask.  A set of macros for manipulating CPU sets is
33       described in CPU_SET(3).
34
35       sched_setaffinity()  sets  the CPU affinity mask of the thread whose ID
36       is pid to the value specified by mask.  If pid is zero, then the  call‐
37       ing  thread  is used.  The argument cpusetsize is the length (in bytes)
38       of the data pointed to by mask.  Normally this argument would be speci‐
39       fied as sizeof(cpu_set_t).
40
41       If  the  thread specified by pid is not currently running on one of the
42       CPUs specified in mask, then that thread is migrated to one of the CPUs
43       specified in mask.
44
45       sched_getaffinity()  writes the affinity mask of the thread whose ID is
46       pid into the cpu_set_t structure pointed to by  mask.   The  cpusetsize
47       argument  specifies  the size (in bytes) of mask.  If pid is zero, then
48       the mask of the calling thread is returned.
49

RETURN VALUE

51       On success, sched_setaffinity() and sched_getaffinity() return  0  (but
52       see "C library/kernel differences" below, which notes that the underly‐
53       ing sched_getaffinity() differs in its return value).  On error, -1  is
54       returned, and errno is set appropriately.
55

ERRORS

57       EFAULT A supplied memory address was invalid.
58
59       EINVAL The  affinity bit mask mask contains no processors that are cur‐
60              rently physically on the system  and  permitted  to  the  thread
61              according  to  any  restrictions  that  may be imposed by cpuset
62              cgroups or the "cpuset" mechanism described in cpuset(7).
63
64       EINVAL (sched_getaffinity()   and,    in    kernels    before    2.6.9,
65              sched_setaffinity())  cpusetsize is smaller than the size of the
66              affinity mask used by the kernel.
67
68       EPERM  (sched_setaffinity()) The calling thread does not have appropri‐
69              ate  privileges.  The caller needs an effective user ID equal to
70              the real user ID or effective user ID of the  thread  identified
71              by  pid,  or  it must possess the CAP_SYS_NICE capability in the
72              user namespace of the thread pid.
73
74       ESRCH  The thread whose ID is pid could not be found.
75

VERSIONS

77       The CPU affinity system calls were introduced in  Linux  kernel  2.5.8.
78       The  system call wrappers were introduced in glibc 2.3.  Initially, the
79       glibc interfaces included a cpusetsize argument, typed as unsigned int.
80       In  glibc  2.3.3,  the  cpusetsize  argument  was removed, but was then
81       restored in glibc 2.3.4, with type size_t.
82

CONFORMING TO

84       These system calls are Linux-specific.
85

NOTES

87       After a call to sched_setaffinity(), the  set  of  CPUs  on  which  the
88       thread  will  actually  run is the intersection of the set specified in
89       the mask argument and the set of CPUs actually present on  the  system.
90       The  system  may  further  restrict the set of CPUs on which the thread
91       runs if the "cpuset" mechanism described in cpuset(7)  is  being  used.
92       These  restrictions  on the actual set of CPUs on which the thread will
93       run are silently imposed by the kernel.
94
95       There are various ways of determining the number of CPUs  available  on
96       the  system, including: inspecting the contents of /proc/cpuinfo; using
97       sysconf(3)  to  obtain  the  values  of  the  _SC_NPROCESSORS_CONF  and
98       _SC_NPROCESSORS_ONLN  parameters; and inspecting the list of CPU direc‐
99       tories under /sys/devices/system/cpu/.
100
101       sched(7) has a description of the Linux scheduling scheme.
102
103       The affinity mask is a per-thread attribute that can be adjusted  inde‐
104       pendently  for  each  of  the  threads  in  a  thread group.  The value
105       returned from a call to gettid(2) can be passed in  the  argument  pid.
106       Specifying  pid as 0 will set the attribute for the calling thread, and
107       passing the value returned from  a  call  to  getpid(2)  will  set  the
108       attribute  for  the main thread of the thread group.  (If you are using
109       the POSIX threads API, then use  pthread_setaffinity_np(3)  instead  of
110       sched_setaffinity().)
111
112       The  isolcpus  boot  option  can be used to isolate one or more CPUs at
113       boot time, so that no processes are scheduled onto those CPUs.  Follow‐
114       ing  the  use  of  this boot option, the only way to schedule processes
115       onto the isolated CPUs is  via  sched_setaffinity()  or  the  cpuset(7)
116       mechanism.   For  further information, see the kernel source file Docu‐
117       mentation/admin-guide/kernel-parameters.txt.  As noted  in  that  file,
118       isolcpus  is  the  preferred  mechanism  of  isolating CPUs (versus the
119       alternative of manually setting the CPU affinity of  all  processes  on
120       the system).
121
122       A  child  created  via fork(2) inherits its parent's CPU affinity mask.
123       The affinity mask is preserved across an execve(2).
124
125   C library/kernel differences
126       This manual page describes the glibc interface  for  the  CPU  affinity
127       calls.   The  actual  system call interface is slightly different, with
128       the mask being typed as unsigned long *, reflecting the fact  that  the
129       underlying implementation of CPU sets is a simple bit mask.
130
131       On  success, the raw sched_getaffinity() system call returns the number
132       of bytes placed copied into the mask buffer; this will be  the  minimum
133       of  cpusetsize  and the size (in bytes) of the cpumask_t data type that
134       is used internally by the kernel to represent the CPU set bit mask.
135
136   Handling systems with large CPU affinity masks
137       The underlying system calls (which represent CPU masks as bit masks  of
138       type  unsigned  long *)  impose  no  restriction on the size of the CPU
139       mask.  However, the cpu_set_t data type used by glibc has a fixed  size
140       of  128  bytes,  meaning that the maximum CPU number that can be repre‐
141       sented is 1023.  If the kernel CPU affinity mask is larger  than  1024,
142       then calls of the form:
143
144           sched_getaffinity(pid, sizeof(cpu_set_t), &mask);
145
146       fail with the error EINVAL, the error produced by the underlying system
147       call for the case where  the  mask  size  specified  in  cpusetsize  is
148       smaller  than  the  size  of  the  affinity  mask  used  by the kernel.
149       (Depending on the system CPU topology, the kernel affinity mask can  be
150       substantially larger than the number of active CPUs in the system.)
151
152       When  working on systems with large kernel CPU affinity masks, one must
153       dynamically allocate the mask argument (see CPU_ALLOC(3)).   Currently,
154       the only way to do this is by probing for the size of the required mask
155       using sched_getaffinity() calls with increasing mask sizes  (until  the
156       call does not fail with the error EINVAL).
157
158       Be  aware that CPU_ALLOC(3) may allocate a slightly larger CPU set than
159       requested (because CPU sets are implemented as bit masks  allocated  in
160       units of sizeof(long)).  Consequently, sched_getaffinity() can set bits
161       beyond the requested allocation size, because the  kernel  sees  a  few
162       additional bits.  Therefore, the caller should iterate over the bits in
163       the returned set, counting those which are set, and stop upon  reaching
164       the value returned by CPU_COUNT(3) (rather than iterating over the num‐
165       ber of bits requested to be allocated).
166

EXAMPLES

168       The program below creates a child process.  The parent and  child  then
169       each  assign  themselves to a specified CPU and execute identical loops
170       that consume some CPU time.  Before terminating, the parent  waits  for
171       the child to complete.  The program takes three command-line arguments:
172       the CPU number for the parent, the CPU number for the  child,  and  the
173       number of loop iterations that both processes should perform.
174
175       As  the  sample runs below demonstrate, the amount of real and CPU time
176       consumed when running the program will  depend  on  intra-core  caching
177       effects and whether the processes are using the same CPU.
178
179       We  first  employ  lscpu(1) to determine that this (x86) system has two
180       cores, each with two CPUs:
181
182           $ lscpu | egrep -i 'core.*:|socket'
183           Thread(s) per core:    2
184           Core(s) per socket:    2
185           Socket(s):             1
186
187       We then time the operation of the example program for three cases: both
188       processes  running on the same CPU; both processes running on different
189       CPUs on the same core; and both processes running on different CPUs  on
190       different cores.
191
192           $ time -p ./a.out 0 0 100000000
193           real 14.75
194           user 3.02
195           sys 11.73
196           $ time -p ./a.out 0 1 100000000
197           real 11.52
198           user 3.98
199           sys 19.06
200           $ time -p ./a.out 0 3 100000000
201           real 7.89
202           user 3.29
203           sys 12.07
204
205   Program source
206
207       #define _GNU_SOURCE
208       #include <sched.h>
209       #include <stdio.h>
210       #include <stdlib.h>
211       #include <unistd.h>
212       #include <sys/wait.h>
213
214       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
215                               } while (0)
216
217       int
218       main(int argc, char *argv[])
219       {
220           cpu_set_t set;
221           int parentCPU, childCPU;
222           int nloops, j;
223
224           if (argc != 4) {
225               fprintf(stderr, "Usage: %s parent-cpu child-cpu num-loops\n",
226                       argv[0]);
227               exit(EXIT_FAILURE);
228           }
229
230           parentCPU = atoi(argv[1]);
231           childCPU = atoi(argv[2]);
232           nloops = atoi(argv[3]);
233
234           CPU_ZERO(&set);
235
236           switch (fork()) {
237           case -1:            /* Error */
238               errExit("fork");
239
240           case 0:             /* Child */
241               CPU_SET(childCPU, &set);
242
243               if (sched_setaffinity(getpid(), sizeof(set), &set) == -1)
244                   errExit("sched_setaffinity");
245
246               for (j = 0; j < nloops; j++)
247                   getppid();
248
249               exit(EXIT_SUCCESS);
250
251           default:            /* Parent */
252               CPU_SET(parentCPU, &set);
253
254               if (sched_setaffinity(getpid(), sizeof(set), &set) == -1)
255                   errExit("sched_setaffinity");
256
257               for (j = 0; j < nloops; j++)
258                   getppid();
259
260               wait(NULL);     /* Wait for child to terminate */
261               exit(EXIT_SUCCESS);
262           }
263       }
264

SEE ALSO

266       lscpu(1), nproc(1), taskset(1), clone(2), getcpu(2), getpriority(2),
267       gettid(2), nice(2), sched_get_priority_max(2),
268       sched_get_priority_min(2), sched_getscheduler(2),
269       sched_setscheduler(2), setpriority(2), CPU_SET(3), get_nprocs(3),
270       pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7), cpuset(7),
271       sched(7), numactl(8)
272

COLOPHON

274       This page is part of release 5.07 of the Linux man-pages project.  A
275       description of the project, information about reporting bugs, and the
276       latest version of this page, can be found at
277       https://www.kernel.org/doc/man-pages/.
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281Linux                             2020-06-09              SCHED_SETAFFINITY(2)
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