I am currently working on solving a positive definite symmetric systems in parallel. The parallel direct solver I used is MUMPS. However, the performance and efficiency of the parallel direct solver are quite disappointing compared to the sequential direct solver (Spooles is the one I used). The number of unknowns are 100k and 200k respectively on the figure.
I am wondering that if the parallel direct solver always has this kind of efficiency or I might make some mistakes on setting up the MUMPS.
Moreover, I also consider using iterative solver from PETSc but I cannot find a good example of using the block jacobi preconditioner, so it will be very much appreciate if someone is able to provide an example for PCBJACOBI and PCASM (I heard that the direct solver can be used in the subdomains and combined by using this PCASM preconditioner).