[Beowulf] Q: IB message rate & large core counts (per node)?

[Brian Dobbins]

Hi Greg,

>   Well, clearly we hope to move more towards hybrid methods -all that's
> old
> > is new again?-
>
> If you want bad performance, sure. If you want good performance, you
> want a device which supports talking to a lot of cores, and then
> multiple devices per node, before you go hybrid. The first two don't
> require changing your code. The last does.
>

> The main reason to use hybrid is if there isn't enough parallelism in
> your code/dataset to use the cores independently.
>

  Actually, it's often *for* performance that we look towards hybrid
methods, albeit in an indirect way - with RAM amounts per node increasing at
the same or lesser rate than cores, and with each MPI task on *some* of our
codes having a pretty hefty memory footprint, using fewer MPI processes and
more threads per task lets us fully utilize nodes that would otherwise have
cores sitting idle due to a lack of available memory.  Sure, we could
rewrite the code to tackle this, too, but in general it seems easier to add
threading in than to rework a complicated parallel decomposition, shared
buffers, etc.

  In a nutshell, even if a hybrid mode *costs* me 10-20% over a direct mode
with an equal number of processors, if it allows me to use 50% more cores in
a node, it works out well for us.  But yes, ignoring RAM constraints,
non-hybrid parallelism tends to be nicer at the moment.


> > But getting back to a technical vein, is the multiplexing an issue due to
> > atomic locks on mapped memory pages?  Or just because each copy reserves
> its
> > own independent buffers?  What are the critical issues?
>
> It's all implementation-dependent. A card might have an on-board
> memory limit, or a limited number of "engines" which process
> messages. Even if it has a option to store some data in main memory,
> often that results in a scalability hit.
>

  Thanks.  I guess I need to read up on quite a bit more and set up some
tests.

  Cheers,
  - Brian
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