Re: MPI octave with hyperthreading?

[Javier Fernandez Baldomero]

Shai Ayal wrote:

> Hi all,
>
> I have a hyper threading Intel CPU. Linux treats it as a 2 processor 
> smp machine. Will I gain anything by using MPI octave to run 2 things 
> in parallel ?

No,I don't think so. LAM/MPI is a message-passing mechanism, most useful 
for distributed-memory systems (clusters and multicomputers). For 
shared-memory (multiprocessors), LAM/MPI works, but nothing is said 
about performance :-)

For SMP machines, many applications will suffer from memory contention. 
That is, your code accesses memory on particular instants of time. When 
you parallelize your application and run it as 2 processes (probably 2 
copies of the same parallel code, ie, an SPMD code), each process tries 
to access memory exactly at the same time -> memory contention. The 
parallel code won't run faster than the sequential one, it gets 
memory-speed-limited. I have tested it and that's what happens to the 
MPITB "Pi" demo code in a cluster of Athlon biprocessors.

If I correctly understood multi-threading, your (single) CPU will try to 
switch threads looking for CPU instructions not requiring the currently 
used functional units/pipelines in the processor. So 2 copies of the 
same SPMD code won't help the CPU to find new instructions, they'll be 
exactly the same instructions in the original thread. Additionally, it 
seems you have to use special compiler switches ?!?
http://www.lam-mpi.org/faq/category3.php3#question1

Of course, a parallel MPI application consisting of 2 different 
codes/processes that access memory at totally disparate moments, will 
probably benefit from an hyperthreaded CPU. But that's not the typical 
Octave-MPITB application.

-javier



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