Hello, I am learning to use PetSc but am just a notice. I have a rather basic question to ask and couldn't not find it on the achieves. I am wanting to test the scalability of a Multigrid solver to the 3D Poisson equation. I found ksp/ex22.c that seems to solve the problem that I'm interested in. I ran it on a large server using different processors. The syntax that I use to run using MPI was ./ex22 -da_grid_x 64 -da_grid_y 64 -da_grid_z 32 I tested it using 2, 4, 8, 16 cpus and found that the time increases. See below. Clearly there is something that I don't understand since the time should be reduced. n wtime --------------------- 2 3m58s 4 3m54s 8 5m51s 16 7m23s Any advice would be greatly appreciated. Best regrads, Francis