sched_setaffinity, sched_getaffinity, CPU_CLR, CPU_ISSET, CPU_SET, CPU_ZERO — set and get a process's CPU affinity mask
#include <sched.h>
int
sched_setaffinity( |
pid_t | pid, |
unsigned int | cpusetsize, | |
cpu_set_t * | mask) ; |
int
sched_getaffinity( |
pid_t | pid, |
unsigned int | cpusetsize, | |
cpu_set_t * | mask) ; |
void
CPU_CLR( |
int | cpu, |
cpu_set_t * | set) ; |
int
CPU_ISSET( |
int | cpu, |
cpu_set_t * | set) ; |
void
CPU_SET( |
int | cpu, |
cpu_set_t * | set) ; |
void
CPU_ZERO( |
cpu_set_t * | set) ; |
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 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 prevents 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 cpu_set_t
structure, a "CPU
set", pointed to by mask
. Four macros are provided
to manipulate CPU sets. CPU_ZERO
() clears a set. CPU_SET
() and CPU_CLR
() respectively add and remove a
given CPU from a set. CPU_ISSET
() tests to see if a CPU is part
of the set; this is useful after sched_getaffinity
() returns. The first
available CPU on the system corresponds to a cpu
value of 0, the next CPU
corresponds to a cpu
value of 1, and so on. The constant CPU_SETSIZE
(1024) specifies a value one
greater than the maximum CPU number that can be stored in a
CPU set.
sched_setaffinity
() sets the
CPU affinity mask of the process whose ID is pid
to the value specified by
mask
. If pid
is zero, then the calling
process is used. The argument cpusetsize
is the length (in
bytes) of the data pointed to by mask
. Normally this argument
would be specified as sizeof(cpu_set_t)
.
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
.
sched_getaffinity
() writes
the affinity mask of the process whose ID is pid
into the cpu_set_t
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.
On 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 bitmask mask
contains no
processors that are physically on the system, or
cpusetsize
is
smaller than the size of the affinity mask used by the
kernel.
The calling process does not have appropriate
privileges. The process calling sched_setaffinity
() needs an
effective user ID equal to the 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.
The CPU affinity system calls were introduced in Linux
kernel 2.5.8. The library interfaces were introduced in glibc
2.3. Initially, the glibc interfaces included a cpusetsize
argument. In glibc
2.3.3, the cpusetsize
argument was removed, but this argument was restored in glibc
2.3.4.
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
.
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 that the fact that the underlying implementation
of CPU sets is a simple bitmask. 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
bitmask.
clone(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), capabilities(7)
sched_setscheduler(2) has a description of the Linux scheduling scheme.
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