# sched_setaffinity

## NAME



           sched_setaffinity, sched_getaffinity - set and get a thread's CPU
           affinity mask
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#define _GNU_SOURCE** /* See feature_test_macros(7) */
           **#include <sched.h>**
    
           **int sched_setaffinity(pid_t** _pid_**, size_t** _cpusetsize_**,**
                                 **const cpu_set_t ***_mask_**);**
           **int sched_getaffinity(pid_t** _pid_**, size_t** _cpusetsize_**,**
                                 **cpu_set_t ***_mask_**);**
    


## 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 _cpu_set_t_ structure, a
           "CPU set", pointed to by _mask_.  A set of macros for manipulating
           CPU sets is described in CPU_SET(3).
    
           **sched_setaffinity**() sets the CPU affinity mask of the thread whose
           ID is _pid_ to the value specified by _mask_.  If _pid_ is zero, then
           the calling thread 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 thread specified by _pid_ is not currently running on one of
           the CPUs specified in _mask_ , then that thread is migrated to one of
           the CPUs specified in _mask_.
    
           **sched_getaffinity**() writes the affinity mask of the thread 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 thread is returned.
    


## RETURN VALUE



           On success, **sched_setaffinity**() and **sched_getaffinity**() return 0
           (but see "C library/kernel differences" below, which notes that
           the underlying **sched_getaffinity**() differs in its return value).
           On failure, -1 is returned, and _errno_ is set to indicate the
           error.
    


## ERRORS



           **EFAULT** A supplied memory address was invalid.
    
           **EINVAL** The affinity bit mask _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
                  _cpuset_ cgroups or the "cpuset" mechanism described in
                  cpuset(7).
    
           **EINVAL**(**sched_getaffinity**() and, before Linux 2.6.9,
                  **sched_setaffinity**()) _cpusetsize_ is smaller than the size of
                  the affinity mask used by the kernel.
    
           **EPERM**(**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 _pid_ , or it must possess the
                  **CAP_SYS_NICE** capability in the user namespace of the thread
                  _pid_.
    
           **ESRCH** The thread whose ID is _pid_ could not be found.
    


## STANDARDS



           Linux.
    


## NOTES



           After a call to **sched_setaffinity**(), the set of CPUs on which the
           thread 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 thread runs if the "cpuset" mechanism described in 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.
    
           There are various ways of determining the number of CPUs available
           on the system, including: inspecting the contents of
           _/proc/cpuinfo_ ; using sysconf(3) to obtain the values of the
           **_SC_NPROCESSORS_CONF** and **_SC_NPROCESSORS_ONLN** parameters; and
           inspecting the list of CPU directories under
           _/sys/devices/system/cpu/_.
    
           sched(7) has a description of the Linux scheduling scheme.
    
           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 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**().)
    
           The _isolcpus_ boot option can be used to isolate one or more CPUs
           at boot time, so that no processes are scheduled onto those CPUs.
           Following the use of this boot option, the only way to schedule
           processes onto the isolated CPUs is via **sched_setaffinity**() or the
           cpuset(7) mechanism.  For further information, see the kernel
           source file _Documentation/admin-guide/kernel-parameters.txt_.  As
           noted in that file, _isolcpus_ is the preferred mechanism of
           isolating CPUs (versus the alternative of manually setting the CPU
           affinity of all processes on the system).
    
           A child created via fork(2) inherits its parent's CPU affinity
           mask.  The affinity mask is preserved across an execve(2).
    
       **C library/kernel differences**
           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
           number of bytes placed copied into the _mask_ buffer; this will be
           the minimum of _cpusetsize_ and the size (in bytes) of the _cpumask_t_
           data type that is used internally by the kernel to represent the
           CPU set bit mask.
    
       **Handling systems with large CPU affinity masks**
           The underlying system calls (which represent CPU masks as bit
           masks of type _unsigned long *_) impose no restriction on the size
           of the CPU mask.  However, the _cpu_set_t_ data type used by glibc
           has a fixed size of 128 bytes, meaning that the maximum CPU number
           that can be represented is 1023.  If the kernel CPU affinity mask
           is larger than 1024, then calls of the form:
    
               sched_getaffinity(pid, sizeof(cpu_set_t), &mask);
    
           fail with the error **EINVAL** , the error produced by the underlying
           system call for the case where the _mask_ size specified in
           _cpusetsize_ is smaller than the size of the affinity mask used by
           the kernel.  (Depending on the system CPU topology, the kernel
           affinity mask can be substantially larger than the number of
           active CPUs in the system.)
    
           When working on systems with large kernel CPU affinity masks, one
           must dynamically allocate the _mask_ argument (see CPU_ALLOC(3)).
           Currently, the only way to do this is by probing for the size of
           the required mask using **sched_getaffinity**() calls with increasing
           mask sizes (until the call does not fail with the error **EINVAL**).
    
           Be aware that CPU_ALLOC(3) may allocate a slightly larger CPU set
           than requested (because CPU sets are implemented as bit masks
           allocated in units of _sizeof(long)_).  Consequently,
           **sched_getaffinity**() can set bits beyond the requested allocation
           size, because the kernel sees a few additional bits.  Therefore,
           the caller should iterate over the bits in the returned set,
           counting those which are set, and stop upon reaching the value
           returned by CPU_COUNT(3) (rather than iterating over the number of
           bits requested to be allocated).
    


## EXAMPLES



           The program below creates a child process.  The parent and child
           then each assign themselves to a specified CPU and execute
           identical loops that consume some CPU time.  Before terminating,
           the parent waits for the child to complete.  The program takes
           three command-line arguments: the CPU number for the parent, the
           CPU number for the child, and the number of loop iterations that
           both processes should perform.
    
           As the sample runs below demonstrate, the amount of real and CPU
           time consumed when running the program will depend on intra-core
           caching effects and whether the processes are using the same CPU.
    
           We first employ lscpu(1) to determine that this (x86) system has
           two cores, each with two CPUs:
    
               $ **lscpu | egrep -i 'core.*:|socket'** ;
               Thread(s) per core:    2
               Core(s) per socket:    2
               Socket(s):             1
    
           We then time the operation of the example program for three cases:
           both processes running on the same CPU; both processes running on
           different CPUs on the same core; and both processes running on
           different CPUs on different cores.
    
               $ **time -p ./a.out 0 0 100000000** ;
               real 14.75
               user 3.02
               sys 11.73
               $ **time -p ./a.out 0 1 100000000** ;
               real 11.52
               user 3.98
               sys 19.06
               $ **time -p ./a.out 0 3 100000000** ;
               real 7.89
               user 3.29
               sys 12.07
    
       **Program source**
    
           #define _GNU_SOURCE
           #include <err.h>
           #include <sched.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/wait.h>
           #include <unistd.h>
    
           int
           main(int argc, char *argv[])
           {
               int           parentCPU, childCPU;
               cpu_set_t     set;
               unsigned int  nloops;
    
               if (argc != 4) {
                   fprintf(stderr, "Usage: %s parent-cpu child-cpu num-loops\n",
                           argv[0]);
                   exit(EXIT_FAILURE);
               }
    
               parentCPU = atoi(argv[1]);
               childCPU = atoi(argv[2]);
               nloops = atoi(argv[3]);
    
               CPU_ZERO(&set);
    
               switch (fork()) {
               case -1:            /* Error */
                   err(EXIT_FAILURE, "fork");
    
               case 0:             /* Child */
                   CPU_SET(childCPU, &set);
    
                   if (sched_setaffinity(0, sizeof(set), &set) == -1)
                       err(EXIT_FAILURE, "sched_setaffinity");
    
                   for (unsigned int j = 0; j < nloops; j++)
                       getppid();
    
                   exit(EXIT_SUCCESS);
    
               default:            /* Parent */
                   CPU_SET(parentCPU, &set);
    
                   if (sched_setaffinity(0, sizeof(set), &set) == -1)
                       err(EXIT_FAILURE, "sched_setaffinity");
    
                   for (unsigned int j = 0; j < nloops; j++)
                       getppid();
    
                   wait(NULL);     /* Wait for child to terminate */
                   exit(EXIT_SUCCESS);
               }
           }
    


* * *