LAM: Why the parallel execution time is longer than sequential time sometimes?

Note that this is the list for supporting LAM/MPI, not
MPICH.  If you have
MPICH-specific questions (and at least part of your
questions below are
MPICH-specific), you should probably ask the MPICH
maintainers -- we can't
really comment on their software.

A general answer to your question is that parallel computing
is not free.
It is not always beneficial to parallelize something because
over the
overhead involved.  In general, you need to have
"enough work" such that the
overhead of parallelization (e.g., network communication
time) needs to be
[much] smaller than your actual wall-clock computation time.

I wrote a web page about this kind of thing a long time ago
-- see if this
helps explain (the project itself is long-since defunct, but
the
explanations are still goo):

    
http://www.osl.iu.edu/~jsquyres/bladeenc/details.php


On 8/28/06 8:14 AM, "Aidaros Dev"
<aidaros.devgmail.com> wrote:

> Im new in MPI and in my lab using Rock cluster 10 nodes
each has 2
> processors, using mpich 1.2.7p1 in
> my Linux platform,
> I compute the execution time by running MPI_Wtime at
begining and end of
> program then subtract them.
> I run the sequential code on the master machine by
$./sequntial
> the execution time is  almost constant from 0.000771
seconds to 0.000789
> I m using two processors for my parallel
> When I run parallel code by $ /opt/mpich/gnu/bin/mpirun
-nolocal -np 2
> -machinefiles machines ./parallel
> the output:
> --------------------------------
> Processor 0 on compute-0-0
> Processpr 1 on compute-0-0
> Execution Time is 0.000503 seconds
> -------------------------------
> This result is acceptable,
> but when i run again $ /opt/mpich/gnu/bin/mpirun
-nolocal -np 2
> -machinefiles machines ./parallel
> the execution time is 0.00129, next run 0.00288,next
run 0.000666,next run
> 0.00199 seconds
> Why the parallel execution time goes up and down, even
sometimes is slower
> than sequential process?.
> Is it shared memory issue?
> 
> Thank for help in advance
> A friend in need Is a friend indeed
> _______________________________________________
> This list is archived at http://www.l
am-mpi.org/MailArchives/lam/


-- 
Jeff Squyres
Server Virtualization Business Unit
Cisco Systems
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