how to set CPU affinity of a particular pthread?
I'd like to specify the cpu-affinity of a particular pthread. All the references I've found so far deal with setting the cpu-affinity of a process (pid_t) not a thread (pthread_t). I tried some experiments passing pthread_t's around and as expected they fail. Am I trying to do something impossible? If not, can you send a pointer please? Thanks a million.
Solution 1:
This is a wrapper I've made to make my life easier. Its effect is that the calling thread gets "stuck" to the core with id core_id
:
// core_id = 0, 1, ... n-1, where n is the system's number of cores
int stick_this_thread_to_core(int core_id) {
int num_cores = sysconf(_SC_NPROCESSORS_ONLN);
if (core_id < 0 || core_id >= num_cores)
return EINVAL;
cpu_set_t cpuset;
CPU_ZERO(&cpuset);
CPU_SET(core_id, &cpuset);
pthread_t current_thread = pthread_self();
return pthread_setaffinity_np(current_thread, sizeof(cpu_set_t), &cpuset);
}
Solution 2:
Assuming linux:
The interface to setting the affinity is - as you've probably already discovered:
int sched_setaffinity(pid_t pid,size_t cpusetsize,cpu_set_t *mask);
Passing 0 as the pid, and it'll apply to the current thread only, or have other threads report their kernel pid with the linux-specific call pid_t gettid(void);
and pass that in as the pid.
Quoting the man page
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().)
Solution 3:
//compilation: gcc -o affinity affinity.c -lpthread
#define _GNU_SOURCE
#include <sched.h> //cpu_set_t , CPU_SET
#include <pthread.h> //pthread_t
#include <stdio.h>
void *th_func(void * arg);
int main(void) {
pthread_t thread; //the thread
pthread_create(&thread,NULL,th_func,NULL);
pthread_join(thread,NULL);
return 0;
}
void *th_func(void * arg)
{
//we can set one or more bits here, each one representing a single CPU
cpu_set_t cpuset;
//the CPU we whant to use
int cpu = 2;
CPU_ZERO(&cpuset); //clears the cpuset
CPU_SET( cpu , &cpuset); //set CPU 2 on cpuset
/*
* cpu affinity for the calling thread
* first parameter is the pid, 0 = calling thread
* second parameter is the size of your cpuset
* third param is the cpuset in which your thread will be
* placed. Each bit represents a CPU
*/
sched_setaffinity(0, sizeof(cpuset), &cpuset);
while (1);
; //burns the CPU 2
return 0;
}
In POSIX environment you can use cpusets to control which CPUs can be used by processes or pthreads. This type of control is called CPU affinity.
The function 'sched_setaffinity' receives pthread IDs and a cpuset as parameter. When you use 0 in the first parameter, the calling thread will be affected