El mar., 23 oct. 2018 a las 13:53, Matthew Knepley (<knep...@gmail.com>) escribió:
> On Tue, Oct 23, 2018 at 6:24 AM Ale Foggia <amfog...@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/> >