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POOpenMPI matrix multiplication
 

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    <p>Hey guys I'm trying to learn OpenMPI in C. I'm having a bit of trouble doing matrix multiplication with this program when I do it, the results are wrong. The program compiles , but I feel that my matrix multiplication algorithm is wrong somewhere. </p> <p>My approach to solving this problem is to use MPI_Scatter to scatter matrix A then transpose matrix B. Then MPI_Scatter matrix B. Once they are scattered I do the calculation for matrix multiplication and Gather the result back to the root process. I'm not sure if I'm missing something, but I don't fully understand Scatter and Gather yet. I know with Send you can send to individual processes and Recv from different processes, but how does this work with Scatter and Gather. Let me know if I made a mistake somewhere in this code. Thanks.</p> <p>My source code:</p> <pre><code>#define N 512 #include &lt;stdio.h&gt; #include &lt;math.h&gt; #include &lt;mpi.h&gt; #include &lt;sys/time.h&gt; print_results(char *prompt, float a[N][N]); int main(int argc, char *argv[]) { int size, rank, blksz, i, j, k; float a[N][N], b[N][N], c[N][N]; char *usage = "Usage: %s file\n"; float row[N][N], col[N][N]; FILE *fd; int portion, lowerbound, upperbound; double elapsed_time, start_time, end_time; struct timeval tv1, tv2; MPI_Init(&amp;argc, &amp;argv); MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank); MPI_Comm_size(MPI_COMM_WORLD, &amp;size); blksz = (int) ceil((double) N / size); /* if (argc &lt; 2) { fprintf (stderr, usage, argv[0]); return -1; } if ((fd = fopen(argv[1], "r")) == NULL) { fprintf(stderr, "%s: Cannot open file %s for reading.\n", argv[0],argv[1]); fprintf(stderr, usage, argv[0]); return -1; } */ //Read input from file for matrices a and b. //The I/O is not timed because this I/O needs //to be done regardless of whether this program //is run sequentially on one processor or in //parallel on many processors. Therefore, it is //irrelevant when considering speedup. if (rank == 0) { for (i = 0; i &lt; N; i++) for (j = 0; j &lt; N; j++) a[i][j] = i + j; for (i = 0; i &lt; N; i++) for (j = 0; j &lt; N; j++) b[i][j] = i + j; /* for (i = 0; i &lt; N; i++) { for (j = i + 1; j &lt; N; j++) { int temp = b[i][j]; b[i][j] = b[j][i]; b[j][i] = temp; } } */ } //TODO: Add a barrier prior to the time stamp. MPI_Barrier(MPI_COMM_WORLD); // Take a time stamp gettimeofday(&amp;tv1, NULL); //TODO: Scatter the input matrices a and b. MPI_Scatter(a, blksz * N, MPI_FLOAT, row, blksz * N, MPI_FLOAT, 0, MPI_COMM_WORLD); MPI_Scatter(b, blksz * N, MPI_FLOAT, col, blksz * N, MPI_FLOAT, 0, MPI_COMM_WORLD); //TODO: Add code to implement matrix multiplication (C=AxB) in parallel. for (i = 0; i &lt; blksz &amp;&amp; rank * blksz + i &lt; N; i++) { for (j = 0; j &lt; N; j++) { c[i][j] = 0.0; for (k = 0; k &lt; N; k++) { c[i][j] += row[i][j] * col[j][k]; } } } //TODO: Gather partial result back to the master process. MPI_Gather(c, blksz * N, MPI_FLOAT, c, blksz * N, MPI_FLOAT, 0, MPI_COMM_WORLD); // Take a time stamp. This won't happen until after the master // process has gathered all the input from the other processes. gettimeofday(&amp;tv2, NULL); elapsed_time = (tv2.tv_sec - tv1.tv_sec) + ((tv2.tv_usec - tv1.tv_usec) / 1000000.0); printf("elapsed_time=\t%lf (seconds)\n", elapsed_time); // print results MPI_Barrier(MPI_COMM_WORLD); print_results("C = ", c); MPI_Finalize(); } print_results(char *prompt, float a[N][N]) { int i, j; printf("\n\n%s\n", prompt); for (i = 0; i &lt; N; i++) { for (j = 0; j &lt; N; j++) { printf(" %.2f", a[i][j]); } printf("\n"); } printf("\n\n"); } </code></pre>
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