[petsc-users] testing scalability for ksp/ex22.c

[Francis Poulin]

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