[mpich-discuss] Unexpected CPU usage shown by Linux top command

[Jeyapandian Kottalam]

Hi 

I see a difference in behavior between MPICH2 versions 1.0.8p1 and 1.2.1p1 (also 1.3) on Linux. Our program alternates between a significant chunk of serial computing and a chunk of parallel computing. During the serial phase the rank > 0 processes are waiting on a mpi_bcast until rank = 0 hits that bcast line. 

With MPICH2 version 1.0.8p1, the Linux 'top' command shows only one process accumulating cpu time; but, with 1.2.1p1 and with 1.3, all processes do. In both cases I am building from exactly the same code base. With print statements I can verify that the rank > 0 processes are indeed waiting on mpi_bcast.

Do you have any possible explanation or suggestion?

Thanks
Kottalam