[Beowulf] programming multicore clusters

[Gerry Creager]

Greg Lindahl wrote:
> On Fri, Jun 15, 2007 at 08:00:24AM -0500, Gerry Creager wrote:
> 
>> For the forseeable future, I'm not developing much but will use the 
>> hybrid SMP/DM capabilities in WRF.  Takes advantage of SMP availability, 
>> and supports message passing between SMP nodes.  I've not used this 
>> capability for benchmarking but it appears to offer significant gains.
> 
> Gerry, why do you think WRF is going to behave any different from MM5
> in this aspect? MM5 slows down in hybrid mode. MPI "takes advantage"
> of SMP. In fact, on big SMPs like Origin and Altix, MPI programs are
> often faster than shared memory or OpenMP programs, unless you've done
> a lot of work on the OpenMP program to improve locality. Why? Because
> you're forced to worry about locality in an MPI program all the time.

Potentially false advertising.  NCAR/MMM are of the opinion that it runs 
faster.  I've not tested it yet...

>> As we get more hybrid HPC capabilities planning for this will be more 
>> important.
> 
> I predict a lot of people will waste a lot of time without seeing a
> benefit.

Likely correct.

gerry
-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University
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