Hi! I need in help! I have a problem: solve the system of equations by the Gauss-Jordan methods with pthreads.
I have a big matrix A (for example 2000x2000), so i must solve Ax = b.
Before I devide matrix to number of threads and each thread must work only with his peace of matrix.
I write it on Ubuntu, and when I compile [g++ main.cpp -o main -lpthread -lm] the it works good(for example, if in 1 thread I get time_of_working = 10 sec, on 2 threads time_of_working = 5.4, i.e. about 2 times faster ), if I compile like this [g++ main.cpp -o main -lpthread -lm -O3] it is only 1.2-1.3 times faster. Why?
I have a big matrix A (for example 2000x2000), so i must solve Ax = b.
Before I devide matrix to number of threads and each thread must work only with his peace of matrix.
Code:
#include <iostream>
#include "synchronize.h"
#include <pthread.h>
using namespace std;
typedef struct _ARGS
{
int thread_count;
int thread_number;
double * a;
double * b;
int n;
int rs;
int re;
} ARGS;
double getfulltime();
void *work (void *ptr)
{
ARGS * arg = (ARGS*) ptr;
int n = arg->n;
//int total_threads = arg->thread_count;
double tmp;
int i, k, j;
for (i = 0; i < n; i++)
{
if (i <= arg->re && i >= arg->rs)
{
tmp = arg->a[i*n + i];
for (k = i; k < n; k++)
arg->a[i*n + k] /= tmp;
arg->b[i] /= tmp;
}
synchronize(arg->thread_count);
if (i > arg->re || i < arg->rs)
{
for (j = arg->rs; j <= arg->re; j++)
{
tmp = arg->a[j * n + i];
for (k = i; k < n; k++)
arg->a[j*n + k] -= arg->a[i*n + k]*tmp;
arg->b[j] -= arg->b[i] * tmp;
}
}
else
{
for (j = arg->rs; j < i; j++)
{
tmp = arg->a[j * n + i];
for (k = i; k < n; k++)
arg->a[j*n + k] -= arg->a[i*n + k] * tmp;
arg->b[j] -= arg->b[i] * tmp;
}
for (j = i + 1; j <= arg->re; j++)
{
tmp = arg->a[j * n + i];
for (k = i; k < n; k++)
arg->a[j*n + k] -= arg->a[i*n + k] * tmp;
arg->b[j] -= arg->b[i] * tmp;
}
}
//synchronize(arg->thread_count);
}
return NULL;
}
double solve_system(int n, double* A, double* b, double**x, int fullthr)
{
pthread_t *threads;
ARGS *args;
int rowPerthr = n/fullthr;
if (!(threads = (pthread_t*) malloc(fullthr * sizeof(pthread_t))))
{
cout<<"Thread allocation error";
return 1;
}
if (!(args = (ARGS*) malloc (fullthr * sizeof(ARGS))))
{
cout<<"ARGS allocation error";
return 1;
}
for (int i = 0; i < fullthr; i++)
{
args[i].thread_number = i;
args[i].thread_count = fullthr;
args[i].n = n;
args[i].a = A;
args[i].b = b;
if(i == fullthr - 1)
{
args[i].rs = i*rowPerthr;
args[i].re = n - 1;
}
else
{
args[i].rs = i*rowPerthr;
args[i].re = (i + 1)*rowPerthr - 1;
}
}
int i;
double t = getfulltime();
for (i = 0; i < fullthr; i++)
pthread_create(threads + i, 0, work, args + i);
for (i = 0; i < fullthr; i++)
pthread_join(threads[i], NULL);
t = getfulltime() - t;
*x = b;
free(threads);
free(args);
return t;
}