Hi, I am working on parallelizing my sequential program using OpenMPI. Although I got performance speedup using many threads, there was slowdown on a small number of threads like 4 threads. I found that it is because getc worked much slower than sequential version. Does OpenMPI override or wrap getc function?
To find the cause, I added mpi.h to the program and compiled it with mpicc. There are two versions on the program; 1. The sequential program without MPI_Init_thread and MPI_Finalize. 2. The sequential program with MPI_Init_thread and MPI_Finalize. Version 2 was 4x slower than version 1. I think it is because of slow getc. Does MPI_Init_thread have some relation with the getc function call? Thank you in advance. Hanjun
