[petsc-users] Strange strong scaling result

[Matthew Knepley]

On Mon, Jul 11, 2022 at 10:34 AM Ce Qin <qince168 at gmail.com> wrote:

> Dear all,
>
> I want to analyze the strong scaling of our in-house FEM code.
> The test problem has about 20M DoFs. I ran the problem using
> various settings. The speedups for the assembly and solving
> procedures are as follows:
>                                    Assembly     Solving
> NProcessors NNodes CoresPerNode
> 1           1      1                    1.0         1.0
> 2           1      2               1.995246    1.898756
>             2      1               2.121401    2.436149
> 4           1      4               4.658187    6.004539
>             2      2               4.666667    5.942085
>             4      1                4.65272    6.101214
> 8           2      4               9.380985   16.581135
>             4      2               9.308575   17.258891
>             8      1               9.314449   17.380612
> 16          2      8              18.575953   34.483058
>             4      4              18.745129   34.854409
>             8      2              18.828393    36.45509
> 32          4      8              37.140626   70.175879
>             8      4              37.166421   71.533865
>
> I don't quite understand this result. Why we can achieve a speedup of
> about 70+ using 32 processors? Could you please help me explain this?
>

We need more data. I would start with the number of iterates that the solver
executes. I suspect this is changing. However, it can be more complicated.
For example, a Block-Jacobi preconditioner gets cheaper as the number of
subdomains increases. Thus we need to know exactly what the solver is doing.

  Thanks,

     Matt


> Thank you in advance.
>
> Best,
> Ce
>
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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