Running in parallel slower than serial?

[kamensky]

Maybe take a look at this tread:

Mumps in parallel slower than in serial

Hey, I had a similar problem which is due to petsc (or in particular its mumps solver) using thread parallelism via OpenMP. Let us say your PC has n threads available. Then, when you run your program in serial it already uses n threads (afaik this is the default). Using mpirun with m instances now lets each of these create another n openmp threads, so you have in total m*n openmp threads, allthough your system can only support n. Therefore, you are slower. You can fix this behaviour setting …