How to assign threads to different cores in C? - c

Question

I created a program that does the addition of 8 numbers using 4 threads, and then the product of the results. How to ensure that each thread is using a separate core for maximum performance gains. I am new to pthreads so I really don't have any idea on how to use it properly. Please provide answers as simple as possible.
My code:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
int global[9];
void *sum_thread(void *arg)
{
int *args_array;
args_array = arg;
int n1,n2,sum;
n1=args_array[0];
n2=args_array[1];
sum = n1*n2;
printf("N1 * N2 = %d\n",sum);
return (void*) sum;
}
void *sum_thread1(void *arg)
{
int *args_array;
args_array = arg;
int n3,n4,sum2;
n3=args_array[2];
n4=args_array[3];
sum2=n3*n4;
printf("N3 * N4 = %d\n",sum2);
return (void*) sum2;
}
void *sum_thread2(void *arg)
{
int *args_array;
args_array = arg;
int n5,n6,sum3;
n5=args_array[4];
n6=args_array[5];
sum3=n5*n6;
printf("N5 * N6 = %d\n",sum3);
return (void*) sum3;
}
void *sum_thread3(void *arg)
{
int *args_array;
args_array = arg;
int n8,n7,sum4;
n7=args_array[6];
n8=args_array[7];
sum4=n7*n8;
printf("N7 * N8 = %d\n",sum4);
return (void*) sum4;
}
int main()
{
int sum3,sum2,sum,sum4;
int prod;
global[0]=9220; global[1]=1110; global[2]=1120; global[3]=2320; global[4]=5100; global[5]=6720; global[6]=7800; global[7]=9290;// the input
pthread_t tid_sum;
pthread_create(&tid_sum,NULL,sum_thread,global);
pthread_join(tid_sum,(void*)&sum);
pthread_t tid_sum1;
pthread_create(&tid_sum1,NULL,sum_thread1,global);
pthread_join(tid_sum1,(void*)&sum2);
pthread_t tid_sum2;
pthread_create(&tid_sum2,NULL,sum_thread2,global);
pthread_join(tid_sum2,(void*)&sum3);
pthread_t tid_sum3;
pthread_create(&tid_sum3,NULL,sum_thread3,global);
pthread_join(tid_sum3,(void*)&sum4);
prod=sum+sum2+sum3+sum4;
printf("The sum of the products is: %d", prod);
return 0;
}

Answer 1

You don't have, don't want and mustn't (I don't know if you somehow you can though) manage hardware resources at such low levels. That's a job for your OS and partially for standard libraries: they have been tested optimized and standardized properly.
I doubt you can do better. If you do what you are saying either you are an expert hardware/OS programmer or you are destroying decades of works :) .
Also consider this fact: your code will not be portable anymore if you could index the cores manually since it depends on the number of cores of your machine.
On the other way multithread programs should work (and even better sometimes) even when having one core. An example is the case where one of the threads doesn't do anything until an event happens: you can make one thread go to "sleep" so that only the other threads use the CPU; then when the event happens it will execute. In a non-multithread program generally polling is used which uses CPU resource to do nothing.
Also #yano said you are multithread program is not really parallel in this case since you are creating the thread and then waiting for it to finish with pthread_join before starting the other threads.