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.

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);
}

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().)

//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

Please find the below example program to cpu-affinity of a particular pthread.

Please add appropriate libs.

double waste_time(long n)
{

    double res = 0;
    long i = 0;
    while (i <n * 200000) {
        i++;
        res += sqrt(i);
    }
    return res;
}
 
void *thread_func(void *param)
{

    unsigned long mask = 1; /* processor 0 */
 
    /* bind process to processor 0 */
    if (pthread_setaffinity_np(pthread_self(), sizeof(mask),
        &mask) <0) {
        perror("pthread_setaffinity_np");
    }
 
    /* waste some time so the work is visible with "top" */
    printf("result: %f\n", waste_time(2000));
 
    mask = 2;   /* process switches to processor 1 now */
    if (pthread_setaffinity_np(pthread_self(), sizeof(mask),
        &mask) <0) {
        perror("pthread_setaffinity_np");
    }
 
    /* waste some more time to see the processor switch */
    printf("result: %f\n", waste_time(2000));
}
 

int main(int argc, char *argv[])
{

    pthread_t my_thread;
 
    if (pthread_create(&my_thread, NULL, thread_func, NULL) != 0) {
        perror("pthread_create");
    }
    pthread_exit(NULL);
}

Compile above program with -D_GNU_SOURCE flag.

The scheduler will change the cpu affinity as it sees fit; to set it persistently please see cpuset in /proc file system.

http://man7.org/linux/man-pages/man7/cpuset.7.html

Or you can write a short program that sets the cpu affinity periodically (every few seconds) with sched_setaffinity