[petsc-users] an ambiguity on a Lanczos solver for a symmetric system -- advice needed

[Matthew Knepley]

On Sat, Apr 19, 2014 at 2:13 PM, Umut Tabak <u.tabak at tudelft.nl> wrote:

> Dear all,
>

For any timing question, we need to see the output f -log_summary. Also, if
you have significant
time in routines you wrote, we need you to create PETSc events for these.

  Matt


> I am experiencing lately some issues with a symmetric Lanczos eigensolver
> in FORTRAN. Basically, I have test code in MATLAB where I am using
> HSL_MA97(MATLAB interface) at the moment
>
> When I program Lanczos iterations in blocks in MATLAB by using HSL_MA97,
> as expected my overall solution time decreases meaning that block solution
> improves the solution efficiency.
>
> Then, to apply the same algorithm on problems on the orders of millions, I
> am transferring the same algorithm to a FORTRAN code but this time with
> MUMPS as the solver then I was expecting the solution time to decrease as
> well, but my overall solution times are increasing when I increase the
> block size.
>
> For a check with MUMPS, I only tried the block solution phase and compared
> 120 single solutions to
>
> 60 solutions by blocks of 2
> 30 solutions by blocks of 4
> 20 solutions by blocks of 6
> 15 solutions by blocks of 8
>
> and saw that the total solution time in comparison to single solves are
> decreasing so I am thinking this is not the source of the problem, I
> believe.
>
> What I am doing is that I am performing a full reorthogonalization in the
> Lanczos loop, which includes some dgemm calls and moreover there are some
> other calls for sparse symmetric matrix vector multiplications from Intel
> MKL.
>
> I could not really understand why the overall solution time is increasing
> with the increase of the block sizes in FORTRAN whereas I was expecting
> even an improvement over my MATLAB code.
>
> Any ideas on what could be going wrong.
>
> Best regards and thanks in advance,
>
> Umut
>



-- 
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
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