[petsc-users] Configuring petsc with MPI on ubuntu quad-core

[Matthew Knepley]

On Wed, Feb 2, 2011 at 5:04 PM, Vijay S. Mahadevan <vijay.m at gmail.com>wrote:

> Matt,
>
> The -with-debugging=1 option is certainly not meant for performance
> studies but I didn't expect it to yield the same cpu time as a single
> processor for snes/ex20 i.e., my runs with 1 and 2 processors take
> approximately the same amount of time for computation of solution. But
> I am currently configuring without debugging symbols and shall let you
> know what that yields.
>
> On a similar note, is there something extra that needs to be done to
> make use of multi-core machines while using MPI ? I am not sure if
> this is even related to PETSc but could be an MPI configuration option
> that maybe either I or the configure process is missing. All ideas are
> much appreciated.


Sparse MatVec (MatMult) is a memory bandwidth limited operation. On most
cheap multicore machines, there is a single memory bus, and thus using more
cores gains you very little extra performance. I still suspect you are not
actually
running in parallel, because you usually see a small speedup. That is why I
suggested looking at -log_summary since it tells you how many processes were
run and breaks down the time.

   Matt


>
> Vijay
>
> On Wed, Feb 2, 2011 at 4:53 PM, Matthew Knepley <knepley at gmail.com> wrote:
> > On Wed, Feb 2, 2011 at 4:46 PM, Vijay S. Mahadevan <vijay.m at gmail.com>
> > wrote:
> >>
> >> Hi,
> >>
> >> I am trying to configure my petsc install with an MPI installation to
> >> make use of a dual quad-core desktop system running Ubuntu. But
> >> eventhough the configure/make process went through without problems,
> >> the scalability of the programs don't seem to reflect what I expected.
> >> My configure options are
> >>
> >> --download-f-blas-lapack=1 --with-mpi-dir=/usr/lib/ --download-mpich=1
> >> --with-mpi-shared=0 --with-shared=0 --COPTFLAGS=-g
> >> --download-parmetis=1 --download-superlu_dist=1 --download-hypre=1
> >> --download-blacs=1 --download-scalapack=1 --with-clanguage=C++
> >> --download-plapack=1 --download-mumps=1 --download-umfpack=yes
> >> --with-debugging=1 --with-errorchecking=yes
> >
> > 1) For performance studies, make a build using --with-debugging=0
> > 2) Look at -log_summary for a breakdown of performance
> >    Matt
> >
> >>
> >> Is there something else that needs to be done as part of the configure
> >> process to enable a decent scaling ? I am only comparing programs with
> >> mpiexec (-n 1) and (-n 2) but they seem to be taking approximately the
> >> same time as noted from -log_summary. If it helps, I've been testing
> >> with snes/examples/tutorials/ex20.c for all purposes with a custom
> >> -grid parameter from command-line to control the number of unknowns.
> >>
> >> If there is something you've witnessed before in this configuration or
> >> if you need anything else to analyze the problem, do let me know.
> >>
> >> Thanks,
> >> Vijay
> >
> >
> >
> > --
> > What most experimenters take for granted before they begin their
> experiments
> > is infinitely more interesting than any results to which their
> experiments
> > lead.
> > -- Norbert Wiener
> >
>



-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
-- Norbert Wiener
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