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

Hong Zhang hzhang at mcs.anl.gov
Mon Apr 21 09:59:42 CDT 2014

Have you tried slepc for eigenvalue problems?
Why do you need mumps in your eigensolver? Shift-and-invert?


On Mon, Apr 21, 2014 at 9:39 AM, Matthew Knepley <knepley at gmail.com> wrote:
> 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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