[MITgcm-support] [EXTERNAL] Better than expected HPC Scaling

[Dimitris Menemenlis]

I like the theory of better cache utilization initially, from 48 to ~250 processors, then degrading @ higher processor count because of communications (and to a lesser extent more grid cells and hence more computations in the overlap regions).

Are you doing any I/O?

> On Oct 12, 2020, at 9:24 PM, Matthew Mazloff <mmazloff at ucsd.edu> wrote:
> 
> Hi Ed
> 
> It depends on the machine you are running on. But its only slightly better at 250 than 48, and 48 does seem a bit low for a job of that size. Maybe you were pushing max memory? It is odd, but I suspect you are on a machine that has very fast interconnects and I/O and 250 is just more efficient. 
> 
> Another possibility is that this is within the machine noise. Or that you were sharing a node when you ran the 48 and 96 job. Or that your tiles were very rectangular for the 48 and 96 core jobs so you had more overlap.
> 
> Matt
> 
> 
>> On Oct 12, 2020, at 9:00 PM, Edward Doddridge <edward.doddridge at utas.edu.au <mailto:edward.doddridge at utas.edu.au>> wrote:
>> 
>> Hi MITgcmers,
>>  
>> As part of an HPC bid I need to provide some scaling information for MITgcm on their cluster. The test configuration is a reentrant channel 600x800x50 grid points, using just the ocean component and some idealised forcing fields. As I increased the core count between 48 and 384 the model scaled better than the theoretical scaling (see attached figure). I’m not complaining that it ran faster, but I was surprised. Any thoughts about what would cause this sort of behaviour? I wondered if it might be something to do with the tiles not fitting in the cache for the low core count simulations. The bid might be more convincing if I can give a plausible explanation for why the model scales better than ideal.
>>  
>> Cheers,
>> Ed
>>  
>>  
>> <image001.png>
>> Edward Doddridge
>> Research Associate and Theme Leader
>> Australian Antarctic Program Partnership (AAPP)
>> Institute for Marine and Antarctic Studies (IMAS)
>> University of Tasmania (UTAS)
>>  
>> doddridge.me <x-msg://76/doddridge.me>
>> 
>> 
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