1) Run with -ksp_view to see what solver options it is really using. By default it is not likely using multigrid like you hope. 2) See http://www.mcs.anl.gov/petsc/documentation/faq.html#computers 3) Run with -log_summary to see where the time is being spent in the different cases Barry On Feb 23, 2012, at 2:16 PM, Francis Poulin wrote:
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