|SCHED_SETAFFINITY(2)||Linux Programmer's Manual||SCHED_SETAFFINITY(2)|
NAMEsched_setaffinity, sched_getaffinity - set and get a process's CPU affinity mask
#define _GNU_SOURCE /* See feature_test_macros(7) */ #include
int sched_setaffinity(pid_t pid , size_t cpusetsize ,
cpu_set_t * mask );
int sched_getaffinity(pid_t pid , size_t cpusetsize ,
cpu_set_t * mask );
DESCRIPTIONA process'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 process (i.e., setting the affinity mask of that process to specify a single CPU, and setting the affinity mask of all other processes to exclude that CPU), it is possible to ensure maximum execution speed for that process. Restricting a process to run on a single CPU also avoids the performance cost caused by the cache invalidation that occurs when a process ceases to execute on one CPU and then recommences execution on a different CPU.
RETURN VALUEOn success, sched_setaffinity() and sched_getaffinity() return 0. On error, -1 is returned, and errno is set appropriately.
- A supplied memory address was invalid.
- The affinity bit mask mask contains no processors that are currently physically on the system and permitted to the process according to any restrictions that may be imposed by the "cpuset" mechanism described in cpuset(7).
- ( sched_getaffinity() and, in kernels before 2.6.9, sched_setaffinity()) cpusetsize is smaller than the size of the affinity mask used by the kernel.
- ( sched_setaffinity()) The calling process does not have appropriate privileges. The caller needs an effective user ID equal to the real user ID or effective user ID of the process identified by pid, or it must possess the CAP_SYS_NICE capability.
- The process whose ID is pid could not be found.
VERSIONSThe 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 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.
CONFORMING TOThese system calls are Linux-specific.
NOTESAfter a call to sched_setaffinity(), the set of CPUs on which the process 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 process runs if the "cpuset" mechanism described in cpuset(7) is being used. These restrictions on the actual set of CPUs on which the process will run are silently imposed by the kernel.
The affinity mask is actually 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 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 attribute 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().)
A child created via fork(2) inherits its parent's CPU affinity mask. The affinity mask is preserved across an execve(2).
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 size (in bytes) of the cpumask_t data type that is used internally by the kernel to represent the CPU set bit mask.
SEE ALSOtaskset(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), pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7), cpuset(7)
COLOPHONThis page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.