[petsc-users] Tuning performance for simple solver

[Dave May]

On 3 June 2016 at 11:37, Michael Becker <
Michael.Becker at physik.uni-giessen.de> wrote:

> Dear all,
>
> I have a few questions regarding possible performance enhancements for the
> PETSc solver I included in my project.
>
> It's a particle-in-cell plasma simulation written in C++, where Poisson's
> equation needs to be solved repeatedly on every timestep.
> The simulation domain is discretized using finite differences, so the
> solver therefore needs to be able to efficiently solve the linear system A
> x = b successively with changing b. The solution x of the previous timestep
> is generally a good initial guess for the solution.
>
> I wrote a class PETScSolver that holds all PETSc objects and necessary
> information about domain size and decomposition. To solve the linear
> system, two arrays, 'phi' and 'charge', are passed to a member function
> solve(), where they are copied to PETSc vectors, and KSPSolve() is called.
> After convergence, the solution is then transferred again to the phi array
> so that other program parts can use it.
>
> The matrix is created using DMDA. An array 'bound' is used to determine
> whether a node is either a Dirichlet BC or holds a charge.
>
> I attached three files, petscsolver.h, petscsolver.cpp and main.cpp, that
> contain a shortened version of the solver class and a set-up to initialize
> and run a simple problem.
>
> Is there anything I can change to generally make the program run faster?
>

Before changing anything, you should profile your code to see where time is
being spent.

To that end, you should compile an optimized build of petsc, link it to you
application and run your code with the option -log_summary. The
-log_summary flag will generate a performance profile of specific
functionality within petsc (KSPSolve, MatMult etc) so you can see where all
the time is being spent.

As a second round of profiling, you should consider registering specific
functionality in your code you think is performance critical.
You can do this using the function PetscLogStageRegister()

http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Profiling/PetscLogStageRegister.html

Check out the examples listed at the bottom of this web page to see how to
log stages. Once you've registered stages, these will appear in the report
provided by -log_summary

Thanks,
  Dave


And, since I'm rather unexperienced with KSP methods, how do I efficiently
> choose PC and KSP? Just by testing every combination?
> Would multigrid be a viable option as a pure solver (-ksp_type preonly)?
>
> Thanks,
> Michael
>
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