.\" Copyright (C) 2002 Robert Love .\" and Copyright (C) 2006 Michael Kerrisk .\" .\" %%%LICENSE_START(GPLv2+_DOC_FULL) .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, see .\" . .\" %%%LICENSE_END .\" .\" 2002-11-19 Robert Love - initial version .\" 2004-04-20 mtk - fixed description of return value .\" 2004-04-22 aeb - added glibc prototype history .\" 2005-05-03 mtk - noted that sched_setaffinity may cause thread .\" migration and that CPU affinity is a per-thread attribute. .\" 2006-02-03 mtk -- Major rewrite .\" 2008-11-12, mtk, removed CPU_*() macro descriptions to a .\" separate CPU_SET(3) page. .\" .TH SCHED_SETAFFINITY 2 2014-08-19 "Linux" "Linux Programmer's Manual" .SH NAME sched_setaffinity, sched_getaffinity \- \ set and get a thread's CPU affinity mask .SH SYNOPSIS .nf .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .B #include .sp .BI "int sched_setaffinity(pid_t " pid ", size_t " cpusetsize , .BI " const cpu_set_t *" mask ); .sp .BI "int sched_getaffinity(pid_t " pid ", size_t " cpusetsize , .BI " cpu_set_t *" mask ); .fi .SH 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 dedicating 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 .I cpu_set_t structure, a "CPU set", pointed to by .IR mask . A set of macros for manipulating CPU sets is described in .BR CPU_SET (3). .BR sched_setaffinity () sets the CPU affinity mask of the thread whose ID is .I pid to the value specified by .IR mask . If .I pid is zero, then the calling thread is used. The argument .I cpusetsize is the length (in bytes) of the data pointed to by .IR mask . Normally this argument would be specified as .IR "sizeof(cpu_set_t)" . If the thread specified by .I pid is not currently running on one of the CPUs specified in .IR mask , then that thread is migrated to one of the CPUs specified in .IR mask . .BR sched_getaffinity () writes the affinity mask of the thread whose ID is .I pid into the .I cpu_set_t structure pointed to by .IR mask . The .I cpusetsize argument specifies the size (in bytes) of .IR mask . If .I pid is zero, then the mask of the calling thread is returned. .SH RETURN VALUE On success, .BR sched_setaffinity () and .BR sched_getaffinity () return 0. On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP .B EFAULT A supplied memory address was invalid. .TP .B EINVAL The affinity bit mask .I mask contains no processors that are currently physically on the system and permitted to the thread according to any restrictions that may be imposed by the "cpuset" mechanism described in .BR cpuset (7). .TP .B EINVAL .RB ( sched_getaffinity () and, in kernels before 2.6.9, .BR sched_setaffinity ()) .I cpusetsize is smaller than the size of the affinity mask used by the kernel. .TP .B EPERM .RB ( sched_setaffinity ()) The calling thread does not have appropriate privileges. The caller needs an effective user ID equal to the real user ID or effective user ID of the thread identified by .IR pid , or it must possess the .B CAP_SYS_NICE capability. .TP .B ESRCH The thread whose ID is \fIpid\fP could not be found. .SH VERSIONS The CPU affinity system calls were introduced in Linux kernel 2.5.8. The system call wrappers were introduced in glibc 2.3. Initially, the glibc interfaces included a .I cpusetsize argument, typed as .IR "unsigned int" . In glibc 2.3.3, the .I cpusetsize argument was removed, but was then restored in glibc 2.3.4, with type .IR size_t . .SH CONFORMING TO These system calls are Linux-specific. .SH NOTES After a call to .BR sched_setaffinity (), the set of CPUs on which the thread will actually run is the intersection of the set specified in the .I 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 .BR 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. .BR sched (7) has a description of the Linux scheduling scheme. .PP The affinity mask is a per-thread attribute that can be adjusted independently for each of the threads in a thread group. The value returned from a call to .BR gettid (2) can be passed in the argument .IR pid . Specifying .I pid as 0 will set the attribute for the calling thread, and passing the value returned from a call to .BR getpid (2) will set the attribute for the main thread of the thread group. (If you are using the POSIX threads API, then use .BR pthread_setaffinity_np (3) instead of .BR sched_setaffinity ().) A child created via .BR fork (2) inherits its parent's CPU affinity mask. The affinity mask is preserved across an .BR execve (2). .SS C library/kernel ABI differences This manual page describes the glibc interface for the CPU affinity calls. The actual system call interface is slightly different, with the .I mask being typed as .IR "unsigned long\ *" , reflecting the fact that the underlying implementation of CPU sets is a simple bit mask. On success, the raw .BR sched_getaffinity () system call returns the size (in bytes) of the .I cpumask_t data type that is used internally by the kernel to represent the CPU set bit mask. .SH SEE ALSO .ad l .nh .BR taskset (1), .BR clone (2), .BR getcpu (2), .BR getpriority (2), .BR gettid (2), .BR nice (2), .BR sched_get_priority_max (2), .BR sched_get_priority_min (2), .BR sched_getscheduler (2), .BR sched_setscheduler (2), .BR setpriority (2), .BR CPU_SET (3), .BR pthread_setaffinity_np (3), .BR sched_getcpu (3), .BR capabilities (7), .BR cpuset (7), .BR sched (7) .SH COLOPHON This page is part of release 3.74 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%http://www.kernel.org/doc/man\-pages/.