[petsc-users] [SLEPc] Number of iterations changes with MPI processes in Lanczos

[Ale Foggia]

El mar., 23 oct. 2018 a las 13:53, Matthew Knepley (<knepley at gmail.com>)
escribió:

> On Tue, Oct 23, 2018 at 6:24 AM Ale Foggia <amfoggia at gmail.com> wrote:
>
>> Hello,
>>
>> I'm currently using Lanczos solver (EPSLANCZOS) to get the smallest real
>> eigenvalue (EPS_SMALLEST_REAL) of a Hermitian problem (EPS_HEP). Those are
>> the only options I set for the solver. My aim is to be able to
>> predict/estimate the time-to-solution. To do so, I was doing a scaling of
>> the code for different sizes of matrices and for different number of MPI
>> processes. As I was not observing a good scaling I checked the number of
>> iterations of the solver (given by EPSGetIterationNumber). I've encounter
>> that for the **same size** of matrix (that meaning, the same problem), when
>> I change the number of MPI processes, the amount of iterations changes, and
>> the behaviour is not monotonic. This are the numbers I've got:
>>
>
> I am sure you know this, but this test is strong scaling and will top out
> when the individual problem sizes become too small (we see this at several
> thousand unknowns).
>

Thanks for pointing this out, we are aware of that and I've been "playing"
around to try to see by myself this behaviour. Now, I think I'll go with
the Krylov-Schur method because is the only solution to the problem of the
number of iterations. With this I think I'll be able to see the individual
problem size effect in the scaling.


>   Thanks,
>
>     Matt
>
>
>>
>> # procs   # iters
>> 960          157
>> 992          189
>> 1024        338
>> 1056        190
>> 1120        174
>> 2048        136
>>
>> I've checked the mailing list for a similar situation and I've found
>> another person with the same problem but in another solver ("[SLEPc] GD is
>> not deterministic when using different number of cores", Nov 19 2015), but
>> I think the solution this person finds does not apply to my problem
>> (removing "-eps_harmonic" option).
>>
>> Can you give me any hint on what is the reason for this behaviour? Is
>> there a way to prevent this? It's not possible to estimate/predict any time
>> consumption for bigger problems if the number of iterations varies this
>> much.
>>
>> Ale
>>
>
>
> --
> 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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