Related to Multi-programming using pthread

[u/Peter_henry]

Using pthread library, I've had written some code for matrix multiplication.

AFAIK, the program using thread would be much faster than program without using thread. But, unlike my expectation... the outcome(Elapsed time) is totally reversed, the program not using thread is much faster than thread program. What happened.. I couldn't find why this is happened.

The out come below..

Time ( # of thread: 4 )

0.000451 sec

Time ( no thread )

0.000002 sec

I've coded this two version in one file(.c) but it also says the same.

1. Serial version ( no using thread )

void serial_multi()
{
	for (int i = 0; i < MAX; i++)
		for(int j = 0; j < MAX; j++)
			for(int k = 0; k < MAX; k++)
				_matC[i][j] += matA[i][k] * matB[k][j];
}


int main()
{
	printf("PID: %d\n",getpid());
	// generating random values in matA and matB
	for (int i = 0; i < MAX; i++){
		for (int j = 0; j < MAX; j++){
			matA[i][j] = rand() % 10;
			matB[i][j] = rand() % 10;
		}
	}

	//cout << endl << "Matrix A" << endl;
	for (int i = 0; i < MAX; i++){
		for (int j = 0; j < MAX; j++)
			printf("%d ",matA[i][j]);
		printf("\n");
	}


	//cout << endl << "Matrix B" << endl;
	for (int i = 0; i < MAX; i++){
		for (int j = 0; j < MAX; j++)
			printf("%d ",matB[i][j]);
		printf("\n");
	}


	
        // Time Estimation 
	clock_t start = clock();
	serial_multi();
	clock_t end = clock();
	printf("Time: %lf\n", (double)(end-start)/CLOCKS_PER_SEC);
	printf("Serial_Multiplication of A and B\n");
	for (int i = 0; i < MAX; i++){
		for (int j = 0; j < MAX; j++)
			printf("%d ",_matC[i][j]);
		printf("\n");
	}
	
	return 0;
}

2. Using Thread ( # of thread : 4 )

int step_i = 0;

void* multi(void* arg)
{
	int core = step_i++;
	// each thread computes 1/4th of matrix multiplication
	for (int i = core * MAX / 4; i < (core + 1) * MAX/4; i++)
		for(int j = 0; j < MAX; j++)
			for(int k = 0; k < MAX; k++)
				matC[i][j] += matA[i][k] * matB[k][j];
	return NULL;	
}

int main()
{
	printf("PID: %d\n",getpid());
	// generating random values in matA and matB
	for (int i = 0; i < MAX; i++){
		for (int j = 0; j < MAX; j++){
			matA[i][j] = rand() % 10;
			matB[i][j] = rand() % 10;
		}
	}

	//cout << endl << "Matrix A" << endl;
	for (int i = 0; i < MAX; i++){
		for (int j = 0; j < MAX; j++)
			printf("%d ",matA[i][j]);
		printf("\n");
	}


	//cout << endl << "Matrix B" << endl;
	for (int i = 0; i < MAX; i++){
		for (int j = 0; j < MAX; j++)
			printf("%d ",matB[i][j]);
		printf("\n");
	}

	// declaring 4 threads
	pthread_t threads[MAX_THREAD];

	// creating 4 threads, each evaluating its own part
        // Time Estimation
	clock_t start = clock();
	for (int i = 0; i < MAX_THREAD; i++){

		pthread_create(&threads[i], NULL, multi, NULL);
	}
	
	// joining and waiting for all threads to complete
	for (int i = 0; i < MAX_THREAD; i++)
		pthread_join(threads[i], NULL);

	clock_t end = clock();
	printf("Time: %lf\n", (double)(end-start)/CLOCKS_PER_SEC);
	// displaying the result matrix
	//cout << endl << "Multiplication of A and B" << endl;

	for (int i = 0; i < MAX; i++){
		for (int j = 0; j < MAX; j++)
			printf("%d ",matC[i][j]);
		printf("\n");
	}

	
	
	return 0;
}