jdtorregrosas - 1 year ago 40

C Question

I need to make the Leibniz algorithm with pthread in c, now I have this code, but at the moment the threads implementation takes the same time of the sequential implementation, I think it is not running concurrently. Can someone see the error.

Thanks!!

`#include<stdio.h>`

#include<math.h>

#include<pthread.h>

#include<stdlib.h>

#define NUM_THREADS 2

#define ITERATIONS 100000000

double result = 0.0;

void *leibniz(void *threadid){

int size = ITERATIONS/NUM_THREADS;

int start = (long)threadid * size;

int end = ((long)threadid+1) * size;

int i;

for(i = start; i<end; i++){

int denom = 2*i+1;

result += pow(-1.0, i) * (1.0/denom);

}

}

int main(){

pthread_t threads[NUM_THREADS];

long t;

int rc;

// CREATE

for(t=0;t<NUM_THREADS;t++){

rc = pthread_create(&threads[t], NULL, leibniz, (void *)t);

if(rc){

printf("ERROR: return code %d\n", rc);

}

}

// JOIN

for(t=0;t<NUM_THREADS;t++){

rc = pthread_join(threads[t], NULL);

if(rc){

printf("ERROR: return code %d\n", rc);

exit(-1);

}

}

printf("Pi %f\n", result*4);

exit(0);

}

Thanks to Jean-François Fabre I did these changes, now it works!

`double result=0.0;`

void *leibniz(void *threadid){

double local = 0.0;

int size = ITERATIONS/NUM_THREADS;

int start = (long)threadid * size;

int end = ((long)threadid+1) * size;

int i;

for(i = start; i<end; i++){

local += (i%2==0 ? 1 : -1) * (1.0/(2*i+1));

}

result += local*4;

}

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Answer Source

I'll attempt to answer.

Even if your application is multithreaded, it's not guaranteed that there's 1 FPU per core. I know little about that but I think that some AMD processors actually *share* the FPU between cores.

Since your loop is basically adding and `pow`

, it's 99% FPU computation, so if the FPU is shared on your computer, it explains the bottleneck.

You could reduce FPU usage by not calling `pow`

just to compute `-1`

or `1`

, which would be a scalar operation, and maybe would make a difference then. just use `-1`

if `i`

is odd, `1`

otherwise, or negate an external 1/-1 variable at each iteration.

Also, in order to avoid race conditions, accumulate the result in a local result, and add it in the end (protecting the addition by a mutex in the end would be even better)

```
double result = 0.0;
void *leibniz(void *threadid){
double local = 0.0;
int size = ITERATIONS/NUM_THREADS;
int start = (long)threadid * size;
int end = ((long)threadid+1) * size;
int i;
for(i = start; i<end; i++){
int denom = 2*i+1;
// using a ternary/scalar speeds up the "pow" computation, multithread or not
local += (i%2 ? -1 : 1) * (1.0/denom);
}
// you may want to protect that addition with a pthread_mutex
// start of critical section
result += local;
// end of critical section
}
```

http://wccftech.com/amd-one-fpu-per-core-design-zen-processors/

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