[mpich-discuss] Single slow mpi process

Dear all, When adding #include "mpi.h" and building a single process with mpi, I find that the single process mpi job runs very much slower than a single thread process built with gcc. i.e. 150 to 200% slower, and run as a standalone executable. That is, for example gcc mycode.c -o mycode.exe ./mycode will run in 100 seconds. Adding #include "mpi.h" to mycode.c mpicc mycode.c -o mycode.exe mpirun -n 1 mycode.exe will take 180 to 400 seconds to run. This varies from system to system on intel, Sparc or IBM based systems. Can anyone explain this offhand ? Has anyone else experienced this ?