[Beowulf] multi-threading vs. MPI

Gerry Creager gerry.creager at tamu.edu
Fri Dec 7 20:56:14 PST 2007

WRF has been under development for 10 years.  It's got an OpenMP flavor, 
an MPI flavor and a hybrid one.  We still don't have all the bugs worked 
out of the hybrid so that it can handle large, high resolution domains 
without being slower than the MPI version.  And, yeah, the OpenMP geeks 
working on this... and the MPI folks, are good.

Hybrid isn't easy and isn't always foolproof.  And, as another thought, 
OpenMP isn't always the best solution to the problem.


richard.walsh at comcast.net wrote:
>  -------------- Original message ----------------------
> From: Toon Knapen <toon.knapen at gmail.com>
>> Greg Lindahl wrote:
>>> In real life (i.e. not HPC), everyone uses message passing between
>>> nodes.  So I don't see what you're getting at.
>> Many on this list suggest that using multiple MPI-processes on one and 
>> the same node is superior to MT approaches IIUC. However I have the 
>> impression that almost the whole industry is looking into MT to benefit 
>> from multi-core without even considering message-passing. Why is that so?
> I think what Greg and others are really saying is that if you have to use a distributed memory
> model (MPI) as a first order response to meet your scalability requirements, then
> the extra coding effort and complexity required to create a hybrid code may not be
> a good performance return on your investment.  If on the other hand you only
> need to scale within a singe SMP node (with cores and sockets on a single
> board growing in number, this returns more performance than in the past), then you
> may be able to avoid using MPI and chose a simpler model like OpenMP.  If you
> have already written an efficient MPI code,  then (with some exceptions) the 
> performance-gain divided by the hybrid coding-effort may seem small.
> Development in an SMP environment is easier.  I know of a number of sights
> that work this way.  The experienced algorithm folks work up the code in 
> OpenMP on say an SGI Altix or Power6 SMP, then they get a dedicated MPI
> coding expert to convert it later for scalable production operation on a cluster.
> In this situation, they do end up with hybrid versions in some cases.  In non-HPC
> or smaller workgroup contexts your production code may not need to be converted.
> Cheers,
> rbw
> --
> "Making predictions is hard, especially about the future."
> Niels Bohr
> --
> Richard Walsh
> Thrashing River Consulting--
> 5605 Alameda St.
> Shoreview, MN 55126
> Phone #: 612-382-4620
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Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
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