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

[Takuya Sekikawa]

I have a few more questions.

Just in case I want to ask.

[Q1]
In general, like ex1, Don't we need any change to source code to
run program with MPI-based system?
in other words, to run SLEPc-based program in MPI-based system,
all I need to do is changing ./configure option (--with-mpi=1, etc)
and re-compiling (without changing source) ?

[Q2]
Can I use intel-compiler (icc/icpc) on MPI-based SLEPc application?
(because intel compiler is really fast so I want to use it on MPI-based
system too)

Takuya

On Wed, 13 Jan 2010 09:50:57 +0100
"Jose E. Roman" <jroman at dsic.upv.es> wrote:

> 
> On 13/01/2010, Takuya Sekikawa wrote:
> 
> > 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?
> 
> Yes. The same example on my desktop computer (Intel Core 2 Duo):
> With -n 1 --> real 33.99
> With -n 2 --> real 21.90
> If you simply have two PCs connected via a slow network, then you cannot expect good speedup. Try in a cluster with fast network.
> On the other hand, a better way to measure the parallel execution time is to edit the source file and put PetscGetTime around the Solve call. 
> 
> > 
> > [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?
> 
> For a 1 million matrix I would suggest to run in parallel on an MPI cluster. However, a single computer might be enough if the matrix is very sparse, you need very few eigenvalues, and/or the system has enough memory (but in that case, be prepared for very long response times, depending on how your problem converges).
> Jose
> 
> > 
> > Thanks,
> > Takuya
> > ---------------------------------------------------------------
> >  Takuya Sekikawa
> >         Mathematical Systems, Inc
> >                    sekikawa at msi.co.jp
> > ---------------------------------------------------------------
> > 
> 

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