[petsc-users] MatSolve in CG

[Jed Brown]

"De Groof, Vincent Frans Maria" <Vincent.De-Groof at uibk.ac.at> writes:

> Dear all,
>
>
> I am writing a few variations on the PCG algorithm. Starting with my own PCG implementation, I noticed that my routine is quite a bit slower (~20%). I think I tracked it down to the MatSolve calls in PCApply. I am using BDDC as the preconditioner.
>
> In the Petsc PCG function, each PCApply calls the MatSolve 3 times. It also calls MatSolve once somewhere between calling KSPSolve and KSPSolve_CG (KSP_SetUp has been called before). So MatSolve is called 1+3*(# iterations) per KSPSolve. In my own PCG variation, the initial MatSolve is not called. But MatSolve is called 4 times in each iteration, so in total 4*(# iterations).
>
> Where does this MatSolve in between KSPSolve and KSPSolve_CG come from? And how can I make my function use less MatSolve's?

I recommend running in a debugger to get the stack trace.  I don't know
what is different between your Krylov implementation and CG, but it
should be obvious when you compare.
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