Question

I’ve managed to get my pthreads program sort of working. Basically I am trying to manually set the affinity of 4 threads such that thread 1 runs on CPU 1, thread 2 runs on CPU 2, thread 3 runs on CPU 3, and thread 4 runs on CPU 4.

After compiling, my code works for a few threads but not others (seems like thread 1 never works) but running the same compiled program a couple of different times gives me different results.

For example:
hao@Gorax:~/Desktop$ ./a.out
Thread 3 is running on CPU 3
pthread_setaffinity_np: Invalid argument
Thread Thread 2 is running on CPU 2
hao@Gorax:~/Desktop$ ./a.out
Thread 2 is running on CPU 2
pthread_setaffinity_np: Invalid argument
pthread_setaffinity_np: Invalid argument
Thread 3 is running on CPU 3
Thread 3 is running on CPU 3
hao@Gorax:~/Desktop$ ./a.out
Thread 2 is running on CPU 2
pthread_setaffinity_np: Invalid argument
Thread 4 is running on CPU 4
Thread 4 is running on CPU 4
hao@Gorax:~/Desktop$ ./a.out
pthread_setaffinity_np: Invalid argument

My question is “Why does this happen? Also, why does the message sometimes print twice?”

Here is the code:

#define _GNU_SOURCE
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <sched.h>
#include <errno.h>

#define handle_error_en(en, msg) \
               do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)

void *thread_function(char *message)
{
    int s, j, number;
    pthread_t thread;
    cpu_set_t cpuset;

    number = (int)message;
    thread = pthread_self();    
    CPU_SET(number, &cpuset);

    s = pthread_setaffinity_np(thread, sizeof(cpu_set_t), &cpuset);
    if (s != 0)
    {
        handle_error_en(s, "pthread_setaffinity_np");
    }

    printf("Thread %d is running on CPU %d\n", number, sched_getcpu());

    exit(EXIT_SUCCESS);
}

int main()
{
    pthread_t thread1, thread2, thread3, thread4;
    int thread1Num = 1;
    int thread2Num = 2;
    int thread3Num = 3;
    int thread4Num = 4;
    int thread1Create, thread2Create, thread3Create, thread4Create, i, temp;

    thread1Create = pthread_create(&thread1, NULL, (void *)thread_function, (char *)thread1Num);
    thread2Create = pthread_create(&thread2, NULL, (void *)thread_function, (char *)thread2Num);
    thread3Create = pthread_create(&thread3, NULL, (void *)thread_function, (char *)thread3Num);
    thread4Create = pthread_create(&thread4, NULL, (void *)thread_function, (char *)thread4Num);

    pthread_join(thread1, NULL);
    pthread_join(thread2, NULL);
    pthread_join(thread3, NULL);
    pthread_join(thread4, NULL);

    return 0;
}

Answer 1

The first CPU is CPU 0 not CPU 1. So you’ll want to change your threadNums:

int thread1Num = 0;
int thread2Num = 1;
int thread3Num = 2;
int thread4Num = 3;

You should initialize cpuset with the CPU_ZERO() macro this way:

CPU_ZERO(&cpuset);
CPU_SET(number, &cpuset);

Also don’t call exit() from a thread as it will stop the whole process with all its threads:

exit(EXIT_SUCCESS);  
return 0; // Use this instead or call pthread_exit()