[MITgcm-support] Choice of workstation CPU for running MITgcm

[Matthew Mazloff]

Hi Christoph

Some answers to your questions. But there are more knowledgable people out there!

The MITgcm scales well and is routinely run on thousands of cores. 
example:
https://people.nas.nasa.gov/~chenze/ECCO/SC05/ecco_sc05.pdf <https://people.nas.nasa.gov/~chenze/ECCO/SC05/ecco_sc05.pdf>

(Obviously if you try to run a small model domain on many cores it will be inefficient.)

In my experience with forward model runs memory isn’t a bottleneck. 

I am not sure what size runs you are talking about, but for runs with great than a few hundred cores I think the bottleneck is primarily with the interconnects and I/O to the NFS. Hopefully people will correct me if I am wrong. 

Matt

> On Apr 9, 2019, at 6:13 AM, Christoph Stappert <cstappert at gmx.de> wrote:
> 
> Hello everyone,
>  
> I am currently building a workstation to run some MITgcm simulations, and I am wondering which of the different CPU models I am considering would be best suited for the task:
>  
> Ryzen 7 1700 (8x 3.0 GHz, dual-channel RAM): A consumer-grade CPU and siginificantly cheaper than the others. However, while it does have ECC, the ECC feature is not officially supported by AMD, so I am reluctant to use this CPU in scientific computing.
>  
> Xeon E-2146G (6x 3.5 GHz, dual-channel RAM): This is the option I am leaning towards at the moment.
>  
> Ryzen Threadripper 1950X (16x 3.4 GHz, quad-channel RAM): More CPU cores than the other two options, but also more expesive. I am wondering, how big would the performance gain actually be in practice?
>  
> I have read in some messages on this list that MITgcm does not scale well with an increasing number of CPU cores and that memory bandwidth is an issue. However, these messages were more than 10 years old, so I am not sure if this still applies to the latest generation of CPUs and to the latest version of the software. I was not able to find any newer messages on hardware recommendations, performance and such.
>  
> My specific questions are:
> - How well does MITgcm scale with an increasing number of CPU cores (4, 8, 16, 32...)? At which point would I stop seeing a significant increase in performance?
> - Is there a bottleneck with memory bandwidth in today's CPUs? Does a higher number of RAM channels significantly increase performance?
> - Are L2 cache and L3 cache a major bottleneck?
> - Does MITgcm benefit from using AVX-512 or other Intel-specific features (since AMD hasn't really been a factor in scientific computing in the last couple of years)?
>  
> Of course, I could just get all the CPU models under consideration and do my own benchmarks, but unforunately, I do not currently have the budget or the time for this. So I was hoping that someone here might have some insights based on their knowledge of the MITgcm code or some personal experience using different kinds of hardware.
>  
> Thank you and kind regards,
>  
> Christoph
>  
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