NAME
schedsetaffinity, schedgetaffinity - set and get a process's CPU affinity mask SYNOPSIS
#define GNUSOURCE /* See featuretestmacros(7) */
#include
int schedsetaffinity(pidt pid, sizet cpusetsize, cpusett *mask); int schedgetaffinity(pidt pid, sizet cpusetsize, cpusett *mask); DESCRIPTION A 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 dedi‐ cating 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. A CPU affinity mask is represented by the cpusett structure, a "CPU set", pointed to by mask. A set of macros for manipulating CPU sets is described in CPUSET(3). schedsetaffinity() sets the CPU affinity mask of the process whose ID is pid to the value specified by mask. If pid is zero, then the call‐ ing process is used. The argument cpusetsize is the length (in bytes) of the data pointed to by mask. Normally this argument would be speci‐ fied as sizeof(cpusett). If the process specified by pid is not currently running on one of the CPUs specified in mask, then that process is migrated to one of the CPUs specified in mask. schedgetaffinity() writes the affinity mask of the process whose ID is pid into the cpusett structure pointed to by mask. The cpusetsize argument specifies the size (in bytes) of mask. If pid is zero, then the mask of the calling process is returned. RETURN VALUE On success, schedsetaffinity() and schedgetaffinity() return 0. On error, -1 is returned, and errno is set appropriately. ERRORS EFAULT A supplied memory address was invalid. EINVAL The affinity bit mask mask contains no processors that are cur‐ rently 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). EINVAL (schedgetaffinity() and, in kernels before 2.6.9, schedsetaffinity()) cpusetsize is smaller than the size of the affinity mask used by the kernel. EPERM (schedsetaffinity()) The calling process does not have appro‐ priate privileges. The caller needs an effective user ID equal to the real user ID or effective user ID of the process identi‐ fied by pid, or it must possess the CAPSYSNICE capability. ESRCH The process whose ID is pid could not be found. 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 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 sizet. CONFORMING TO
These system calls are Linux-specific. NOTES After a call to schedsetaffinity(), 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. schedsetscheduler(2) has a description of the Linux scheduling scheme.
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 pthreadsetaffinitynp(3) instead of schedsetaffinity().) 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 suc‐ cess, the raw schedgetaffinity() system call returns the size (in bytes) of the cpumaskt data type that is used internally by the kernel to represent the CPU set bit mask. SEE ALSO taskset(1), clone(2), getcpu(2), getpriority(2), gettid(2), nice(2), schedgetprioritymax(2), schedgetprioritymin(2), schedgetscheduler(2), schedsetscheduler(2), setpriority(2), CPUSET(3), pthreadsetaffinitynp(3), schedgetcpu(3), capabilities(7), cpuset(7) COLOPHON
This 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/.
Linux 2013-02-11 SCHEDSETAFFINITY(2)