Dear all,
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
