[petsc-users] On what condition is useful MPI-based solution?

[Takuya Sekikawa]

Dear SLEPc/PETSc team,

I tried to run SLEPc's samples program ex1 in MPI-based multi-PC
environment. (1-PC vs 2-PCs)

(ex)
[running ex1 in only 1 PC]
----------------------------------------------------------
$ time -p mpiexec -n 1 ./ex1 -n 4000 -eps_max_it 10000
...
real 76.2
----------------------------------------------------------
says, ex1 took 76.2 seconds.

next, I run same sample in 2 PCs environment.

[running ex1 in 2 PCs]
----------------------------------------------------------
$ time -p mpiexec -n 2 ./ex1 -n 4000 -eps_max_it 10000
...
real 265.54
----------------------------------------------------------
I got 265.54 seconds. (slower than single PC)

[Q1]
Can ex1 sample speed up with MPI?, if so, generally on what condition?

[Q2]
Generally, Is MPI only useful in very large matrix?
Now I have to solve eigenvalue problem of 1M x 1M matrix,
Should I use MPI-based system?

Thanks,
Takuya
---------------------------------------------------------------
  Takuya Sekikawa
         Mathematical Systems, Inc
                    sekikawa at msi.co.jp
---------------------------------------------------------------