sched_setaffinity() - Unix, Linux System Call
sched_setaffinity, sched_getaffinity, CPU_CLR, CPU_ISSET, CPU_SET, CPU_ZERO
- set and get a processs CPU affinity mask
int sched_setaffinity(pid_t pid, unsigned int cpusetsize,
int sched_getaffinity(pid_t pid, unsigned int cpusetsize,
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 processs 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.
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_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
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.
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
pid is zero, then the calling process is used.
cpusetsize is the length (in bytes) of the data pointed to by
mask. Normally this argument would be specified as
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
sched_getaffinity() writes the affinity mask of the process whose ID is
pid into the
cpu_set_t structure pointed to by
cpusetsize argument specifies the size (in bytes) of
pid is zero, then the mask of the calling process is returned.
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,
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
The process whose ID is pid could not be found.
These system calls are Linux specific.
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
can be passed in the argument
A child created via
inherits its parents CPU affinity mask.
The affinity mask is preserved across an
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.
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
In glibc 2.3.2, the
cpusetsize argument was removed, but this argument was restored in glibc 2.3.4.
has a description of the Linux scheduling scheme.