[petsc-users] Suspect Poor Performance of Petsc

[Ivano Barletta]

Dear Petsc users

My aim is to replace the linear solver of an ocean model with Petsc, to see
if is
there place for improvement of performances.

The linear system solves an elliptic equation, and the former solver is a
Preconditioned Conjugate Gradient, with a simple diagonal preconditioning.
The size of the matrix is roughly 27000

Prior to nest Petsc into the model, I've built a simple test case, where
the same system is solved by the two of the methods

I've noticed that, compared to the former solver (pcg), Petsc performance
results are quite disappointing

Pcg does not scale that much, but its solution time remains below
4-5e-2 seconds.
Petsc solution time, instead, the more cpu I use the more increases
(see output of -log_view in attachment ).

I've only tried to change the ksp solver ( gmres, cg, and bcgs with no
improvement) and preconditioning is the default of Petsc. Maybe these
options don't suit my problem very well, but I don't think this alone
justifies this strange behavior

I've tried to provide d_nnz and o_nnz for the exact number of nonzeros in
the
Preallocation phase, but no gain even in this case.

At this point, my question is, what am I doing wrong?

Do you think that the problem is too small for the Petsc to
have any effect?

Thanks in advance
Ivano
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