[Beowulf] problem : mpi dynamic scheduling ??

Mark Hahn hahn at physics.mcmaster.ca
Thu Jul 7 17:38:38 EDT 2005


> > in other words, large-tight apps are incompatible with meaningful
> > load-balancing.  
> 
> Mark,
> 
> There are a couple of examples of tightly coupled apps that do either
> static or dynamic load balancing, but in most cases this load
> balancing was programmed in by the programmer above the MPI level. For
> example, ocean codes don't have to compute on land, so you can static
> load balance by using different-sized rectangles for your data
> decomposition.

absolutely - I meant "load balance" in the context of the original poster,
who was talking about openmosix moving programs to less-loaded nodes.
that is, process load-balancing, rather than domain-decomp-type balancing.

> A system that does this invisibly is Charm++, used by
> namd, a molecular dynamics code. In namd's case the workload at a
> given point varies over time.

indeed - my experience is that any *serious* simulation-type code 
needs to do this kind of data-load balancing (adaptive domain decomposition).
often, the picture gets really interesting when you make timesteps 
adaptive as well as spatial dimensions ;)

really, my comment is mainly to emphasize that for intensive MPI apps
(which means significant in size and fairly tightly-coupled)
you have to avoid jitter at all costs.  here's a BEAUTIFUL paper:

http://www.sc-conference.org/sc2003/paperpdfs/pap301.pdf

regards, mark hahn.

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