man2/sched_setaffinity.2

   1 .\" man2/sched_setaffinity.2 - sched_setaffinity and sched_getaffinity man page
   2 .\"
   3 .\" Copyright (C) 2002 Robert Love
   4 .\" and Copyright (C) 2006 Michael Kerrisk
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  26 .\"
  27 .\" 2002-11-19 Robert Love <rml@tech9.net> - initial version
  28 .\" 2004-04-20 mtk - fixed description of return value
  29 .\" 2004-04-22 aeb - added glibc prototype history
  30 .\" 2005-05-03 mtk - noted that sched_setaffinity may cause thread
  31 .\"     migration and that CPU affinity is a per-thread attribute.
  32 .\" 2006-02-03 mtk -- Major rewrite
  33 .\" 2008-11-12, mtk, removed CPU_*() macro descriptions to a
  34 .\" separate CPU_SET(3) page.
  35 .\"
  36 .TH SCHED_SETAFFINITY 2 2013-02-11 "Linux" "Linux Programmer's Manual"
  37 .SH NAME
  38 sched_setaffinity, sched_getaffinity \- \
  39 set and get a process's CPU affinity mask
  40 .SH SYNOPSIS
  41 .nf
  42 .BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
  43 .B #include <sched.h>
  44 .sp
  45 .BI "int sched_setaffinity(pid_t " pid ", size_t " cpusetsize ,
  46 .BI "                      cpu_set_t *" mask );
  47 .sp
  48 .BI "int sched_getaffinity(pid_t " pid ", size_t " cpusetsize ,
  49 .BI "                      cpu_set_t *" mask );
  50 .fi
  51 .SH DESCRIPTION
  52 A process's CPU affinity mask determines the set of CPUs on which
  53 it is eligible to run.
  54 On a multiprocessor system, setting the CPU affinity mask
  55 can be used to obtain performance benefits.
  56 For example,
  57 by dedicating one CPU to a particular process
  58 (i.e., setting the affinity mask of that process to specify a single CPU,
  59 and setting the affinity mask of all other processes to exclude that CPU),
  60 it is possible to ensure maximum execution speed for that process.
  61 Restricting a process to run on a single CPU also avoids
  62 the performance cost caused by the cache invalidation that occurs
  63 when a process ceases to execute on one CPU and then
  64 recommences execution on a different CPU.
  65
  66 A CPU affinity mask is represented by the
  67 .I cpu_set_t
  68 structure, a "CPU set", pointed to by
  69 .IR mask .
  70 A set of macros for manipulating CPU sets is described in
  71 .BR CPU_SET (3).
  72
  73 .BR sched_setaffinity ()
  74 sets the CPU affinity mask of the process whose ID is
  75 .I pid
  76 to the value specified by
  77 .IR mask .
  78 If
  79 .I pid
  80 is zero, then the calling process is used.
  81 The argument
  82 .I cpusetsize
  83 is the length (in bytes) of the data pointed to by
  84 .IR mask .
  85 Normally this argument would be specified as
  86 .IR "sizeof(cpu_set_t)" .
  87
  88 If the process specified by
  89 .I pid
  90 is not currently running on one of the CPUs specified in
  91 .IR mask ,
  92 then that process is migrated to one of the CPUs specified in
  93 .IR mask .
  94
  95 .BR sched_getaffinity ()
  96 writes the affinity mask of the process whose ID is
  97 .I pid
  98 into the
  99 .I cpu_set_t
 100 structure pointed to by
 101 .IR mask .
 102 The
 103 .I cpusetsize
 104 argument specifies the size (in bytes) of
 105 .IR mask .
 106 If
 107 .I pid
 108 is zero, then the mask of the calling process is returned.
 109 .SH RETURN VALUE
 110 On success,
 111 .BR sched_setaffinity ()
 112 and
 113 .BR sched_getaffinity ()
 114 return 0.
 115 On error, \-1 is returned, and
 116 .I errno
 117 is set appropriately.
 118 .SH ERRORS
 119 .TP
 120 .B EFAULT
 121 A supplied memory address was invalid.
 122 .TP
 123 .B EINVAL
 124 The affinity bit mask
 125 .I mask
 126 contains no processors that are currently physically on the system
 127 and permitted to the process according to any restrictions that
 128 may be imposed by the "cpuset" mechanism described in
 129 .BR cpuset (7).
 130 .TP
 131 .B EINVAL
 132 .RB ( sched_getaffinity ()
 133 and, in kernels before 2.6.9,
 134 .BR sched_setaffinity ())
 135 .I cpusetsize
 136 is smaller than the size of the affinity mask used by the kernel.
 137 .TP
 138 .B EPERM
 139 .RB ( sched_setaffinity ())
 140 The calling process does not have appropriate privileges.
 141 The caller needs an effective user ID equal to the real user ID
 142 or effective user ID of the process identified by
 143 .IR pid ,
 144 or it must possess the
 145 .B CAP_SYS_NICE
 146 capability.
 147 .TP
 148 .B ESRCH
 149 The process whose ID is \fIpid\fP could not be found.
 150 .SH VERSIONS
 151 The CPU affinity system calls were introduced in Linux kernel 2.5.8.
 152 The system call wrappers were introduced in glibc 2.3.
 153 Initially, the glibc interfaces included a
 154 .I cpusetsize
 155 argument, typed as
 156 .IR "unsigned int" .
 157 In glibc 2.3.3, the
 158 .I cpusetsize
 159 argument was removed, but was then restored in glibc 2.3.4, with type
 160 .IR size_t .
 161 .SH CONFORMING TO
 162 These system calls are Linux-specific.
 163 .SH NOTES
 164 After a call to
 165 .BR sched_setaffinity (),
 166 the set of CPUs on which the process will actually run is
 167 the intersection of the set specified in the
 168 .I mask
 169 argument and the set of CPUs actually present on the system.
 170 The system may further restrict the set of CPUs on which the process
 171 runs if the "cpuset" mechanism described in
 172 .BR cpuset (7)
 173 is being used.
 174 These restrictions on the actual set of CPUs on which the process
 175 will run are silently imposed by the kernel.
 176
 177 .BR sched_setscheduler (2)
 178 has a description of the Linux scheduling scheme.
 179 .PP
 180 The affinity mask is actually a per-thread attribute that can be
 181 adjusted independently for each of the threads in a thread group.
 182 The value returned from a call to
 183 .BR gettid (2)
 184 can be passed in the argument
 185 .IR pid .
 186 Specifying
 187 .I pid
 188 as 0 will set the attribute for the calling thread,
 189 and passing the value returned from a call to
 190 .BR getpid (2)
 191 will set the attribute for the main thread of the thread group.
 192 (If you are using the POSIX threads API, then use
 193 .BR pthread_setaffinity_np (3)
 194 instead of
 195 .BR sched_setaffinity ().)
 196
 197 A child created via
 198 .BR fork (2)
 199 inherits its parent's CPU affinity mask.
 200 The affinity mask is preserved across an
 201 .BR execve (2).
 202
 203 This manual page describes the glibc interface for the CPU affinity calls.
 204 The actual system call interface is slightly different, with the
 205 .I mask
 206 being typed as
 207 .IR "unsigned long *" ,
 208 reflecting the fact that the underlying implementation of CPU
 209 sets is a simple bit mask.
 210 On success, the raw
 211 .BR sched_getaffinity ()
 212 system call returns the size (in bytes) of the
 213 .I cpumask_t
 214 data type that is used internally by the kernel to
 215 represent the CPU set bit mask.
 216 .SH SEE ALSO
 217 .ad l
 218 .nh
 219 .BR taskset (1),
 220 .BR clone (2),
 221 .BR getcpu (2),
 222 .BR getpriority (2),
 223 .BR gettid (2),
 224 .BR nice (2),
 225 .BR sched_get_priority_max (2),
 226 .BR sched_get_priority_min (2),
 227 .BR sched_getscheduler (2),
 228 .BR sched_setscheduler (2),
 229 .BR setpriority (2),
 230 .BR CPU_SET (3),
 231 .BR pthread_setaffinity_np (3),
 232 .BR sched_getcpu (3),
 233 .BR capabilities (7),
 234 .BR cpuset (7)