how to set CPU affinity of a particular pthread? - multithreading

Question

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.

Answer 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);
}

Answer 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().)

Answer 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

Answer 4

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.

Answer 5

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