[DFTB-Plus-User] OpenMP scaling questions

[Ben Hourahine]

Hello Chi-Ta,

or a system this small, 20 cores is probably above the point where you
gain from parallelism in the eigensolver. Have you tested for lower
numbers (4, 8)?


The ideal parallel scaling should look like


https://dftbplus-recipes.readthedocs.io/en/master/parallel/amdahl.html#amdahl-s-law


but this ignores various effects like efficiency for sub-problems
fitting into various levels of memory hierarchy. This way be in part why
you are seeing the 30 core anomaly.


The asterisks in the output is a known problem with format breaking

https://github.com/dftbplus/dftbplus/issues/182


Regards


Ben





On 30/10/18 03:55, Yang, Chi-Ta wrote:
>
> Greetings,
>
>
> I am using dftbplus-18.2, and testing the "core" and "calculation
> time" upon a system as below. 
>
>
> Test system details:
>
> - 809 atoms and periodic 
> - Gamma point k-point sampling
>
> I was running on jobs with 20, 30, and 40 cores, but the elapsed time
> are comparable. 
> 20 cores job:  13:36:16 hours
> 30 cores job:  15:14:54 hours
> 40 cores job:  12:12:32 hour
>
> The outputs show the OpenMP threads were as expected, but the 40-cores
> job didn't get the benefit as compared to 20-cores job. 
>
> Could you please help why there is no scaling?
>
>
> BTW,  the running time shows ***** as below. Is there a way to show
> the full digits. 
> --------------------------------------------------------------------------------
> DFTB+ running times                          cpu [s]             wall
> clock [s]
> --------------------------------------------------------------------------------
> Sparse H0 and S build              16557.11  (  1.6%)    423.90  ( -0.2%)
> SCC                                    + ********  ( 87.8%)  ******** 
> (104.3%)
> Diagonalisation                        ********  ( 63.4%)  ******** 
> (108.9%)
>
>
> Thanks a lot,
> Chi-Ta Yang
>
>
>
>
>  
>
>
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-- 
      Dr. B. Hourahine, SUPA, Department of Physics,
    University of Strathclyde, John Anderson Building,
            107 Rottenrow, Glasgow G4 0NG, UK.
    +44 141 548 2325, benjamin.hourahine at strath.ac.uk

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