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Subject: Re: [OMPI devel] Comm_spawn limits
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2008-10-27 18:14:10

On Oct 27, 2008, at 5:52 PM, Andreas Schäfer wrote:

> I don't know any implementation details, but is making a 16-bit
> counter a 32-bit counter really so much harder than this fancy
> (overengineered? ;-) ) table construction? The way I see it, this
> table which might become a real mess if there are multiple
> MPI_Comm_spawn issued simultaneously in different communicators.
> (Would
> that be legal MPI?)

FWIW, all the spawns are proxied back to the HNP (i.e., mpirun), so
there would only be a need for 1 table. I don't think that a simple
table lookup is overengineered. :-) It's a simple solution to the
"need a global ID" issue. By limiting the size of the table, you can
avoid scalability issues as MPI jobs are being run on more and more
cores (e.g., growing without bound, particularly for 99% of the apps
out there that never call comm_spawn).

We actually went down to 16 bits recently (it used to be 32) as one
item toward reducing the memory footprint of MPI processes (and mpirun
and the orted's), particularly when running very large scale jobs. So
while increasing this one value back to 32 bits may not be tragic, it
would be nice to keep it down as 16 bits (IMHO).

Regardless of how big the value is (8, 16, 32, 64...) you still need a
unique value for comm_spawn. Therefore, some kind of duplicate
detection mechanism is needed. If you increase the size of the
container, you decrease the probability of collision, but it can still
happen. And since machines are growing in size and # of cores, it
could just delay the probability of collision until someone runs on a
big enough machine. Regardless, I'd prefer to fix it the Right way
rather than rely on probability to prevent a problem. In my
experience, "that could *never* happen!" is just an invitation for a
disaster, even if it's 1-5 years in the future. (didn't someone say
that we'd never need more than 640k of RAM? :-) )

Just my IMHO, of course... (and I'm not the guy writing the code!) :-)

Jeff Squyres
Cisco Systems