1SCHED_SETAFFINITY(2)       Linux Programmer's Manual      SCHED_SETAFFINITY(2)
2
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       int sched_getaffinity(pid_t pid, size_t cpusetsize,
16                             cpu_set_t *mask);
17

DESCRIPTION

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

RETURN VALUE

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

ERRORS

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

VERSIONS

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

CONFORMING TO

83       These system calls are Linux-specific.
84

NOTES

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

EXAMPLES

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

SEE ALSO

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

COLOPHON

273       This page is part of release 5.13 of the Linux man-pages project.  A
274       description of the project, information about reporting bugs, and the
275       latest version of this page, can be found at
276       https://www.kernel.org/doc/man-pages/.
277
278
279
280Linux                             2021-03-22              SCHED_SETAFFINITY(2)
Impressum