Timing in MPI

1. Timing in MPI Tarik Booker MPI Presentation May 7, 2003

2. What we will cover… • How to time programs • Sample Programs • My Samples • MPI Pi Program

3. How to Time Programs • Very Easy • Simply use function MPI_Wtime()

4. MPI_Wtime() • Null input • Returns a Long Float (double) • Not like UNIX “clock” function • MPI_Wtime() is somewhat arbitrary • Starts depending on node

5. How to Time Code • Must use time blocks • Start time • End time • Time for your code is simply end_time – start_time.

6. Example #include<stdio.h> #include<mpi.h> main(int argc, char **argv) { int size, node; double start, end; MPI_Init(&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &node); MPI_Comm_size(MPI_COMM_WORLD, &size); start = MPI_Wtime(); if(node==0) { printf(" Hello From Master. Time = %lf \n", MPI_Wtime() - start); //Count number of ticks } else { printf("Hello From Slave #%d %lf \n", node, (MPI_Wtime() - start)); } MPI_Finalize(); }

7. Example (2) #include<stdio.h> #include<mpi.h> main(int argc, char **argv) { int size, node; int serial_counter = 0; double start, end; MPI_Init(&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &node); MPI_Comm_size(MPI_COMM_WORLD, &size); MPI_Barrier(MPI_COMM_WORLD); start = MPI_Wtime(); MPI_Barrier(MPI_COMM_WORLD); if(node==0) { printf("Hello From Node #%d %lf \n", node, (MPI_Wtime() - start)); } else { printf("Hello From Slave #%d #%lf \n", node, (MPI_Wtime() - start)); } MPI_Finalize(); }

8. Pi Example • Uses MPI to compute value of Pi using formula:

9. Pi Example (2) start_time = MPI_Wtime(); MPI_Bcast(&n, 1, MPI_INT, host_rank, MPI_COMM_WORLD); end_time = MPI_Wtime(); communication_time = end_time - start_time;

10. Pi Example (3) start_time = MPI_Wtime(); h = 1.0 / (double) n; sum = 0.0; for (i = my_rank + 1; i <= n; i += pool_size) { x = h * ((double)i - 0.5); sum += f(x); } mypi = h * sum; end_time = MPI_Wtime(); computation_time = end_time - start_time;

11. Pi Example (4) start_time = MPI_Wtime(); MPI_Reduce(&mypi, &pi, 1, MPI_DOUBLE, MPI_SUM, host_rank, MPI_COMM_WORLD); end_time = MPI_Wtime(); communication_time = communication_time + end_time - start_time; http://dune.mcs.kent.edu/~farrell/dist/ref/mpitut/mpi.html