[mesa-users] MESA on intel xeon phi

[Bill Paxton]

Hi Tassos,

Good to hear from you.  Hope all is well in Geneva.  ;D

Rob Farmer is working on MESA + Phi, so he may want to chime in with more details.

My focus is to use the Phi to prepare for workstation architectures of 5+ years in the future.
I'm betting that the standard workstation in 5+ years will have a processor that has 100's of cores.
It will be similar to the Phi, but it won't be a coprocessor -- it will the the main processor itself.

So my plan is to work with the Phi now to learn how to use 100's of cores effectively.  I don't want to get
distracted by questions of how to use a coprocessor efficiently, since I think that is a short term issue.
I want to concentrate on using the many core architecture, and the Phi is just a way to get started now.

That means the Phi itself may not be a short term practical tool for improved performance;
the individual cores on the Phi are about 3x slower than on the host and the overhead of running
as a coprocessor may slow it down too much to compete with the host.   But it can still be very useful
as a development tool to see how/if we can restructure MESA to use over 100 hardware threads.

Corollary 1: I don't care at this time about speed on Phi vs speed on host.  That will come later.
For now, I only care about speed up on the Phi as we go from 8 or 10 threads to 80 or 100.
That is the measure of how well we are doing going from multi-core to many-core.
And that's where the long term payoff will come.  And, btw, we have a lot of work to do!
I don't want to go into precise numbers yet since we are still working on it (Rob in particular).

You can of course run MESA in native mode on the Phi -- that's what we're doing for testing.
(Rob's the expert.)   It will currently be several times slower than the host, but that's still useful
if the alternative is doing nothing with the Phi at all!   One caveat -- crlibm doesn't work on the Phi
in the sense that we cannot guarantee bit-for-bit identical results.   There is an effort
in progress (by the original crlibm group) to create a successor that will support the Phi,
but it isn't ready yet.   So if you run things on the Phi, you can't expect bit-for-bit reproducibility
on a workstation.

Corollary 2: I don't want to get distracted by issues of supporting MESA on the Phi now.
We aren't to that stage yet.   So if you decide to try MESA with the Phi, please keep that in mind.

Cheers,
Bill



On Nov 23, 2014, at 3:29 AM, Anastasios Fragkos wrote:

> Dear Bill and the rest of the MESA community,
> 
> I remember from a past email in this list that Bill, and maybe some other users, have been running MESA on a Xeon Phi co-processor. I will soon buy a new workstation and I am considering adding a Xeon Phi, so I was wondering if anyone can share their experience on running MESA on Xeon Phi. For example, what kind of performance do you see compared to the host cpu? Can you run more than one MESA simulations on the same Xeon Phi simultaneously? Are there any limitations that we should be aware of? Are there any MESA updates planned in the near/mid future from the developers side  that would be relevant for Xeon Phi?
> 
> Thank you!
> Tassos
> 
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