[petsc-users] Performance of Fieldsplit PC

[Matthew Knepley]

On Tue, Nov 7, 2017 at 8:43 AM, Bernardo Rocha <
bernardomartinsrocha at gmail.com> wrote:

> Hello everyone,
>
> I have a general question about the performance of the PCFieldSplit
> that I'm not sure if I understood properly.
>
> Consider a simple Poisson problem discretized by FEM into a system Ax=b
> which is then solved by CG and Jacobi.
>
> Then, I create a "vectorial Poisson" problem by simply adding another block
> of this problem to create a block-like version of it.
> Something like
> [ [A, 0]
>   [0, A]]
> then I create a PCFieldSplit with CG and Jacobi for each block.
>
> Either with additive or multiplicative fieldsplit, the PC is much better
> and solves it
> with fewer iterations than the scalar case. However, the execution time
> taken by
> the PCFieldSplit is much bigger than the simple Jacobi for the scalar case.
>
> (From -log_view I see all the time difference in PCApply)
>
> Why is this happening?
>

1) This is block-Jacobi, why not use PCBJACOBI? Is it because you want to
select rows?

2) We cannot tell anything without knowing how many iterates were used

  -ksp_monitor_true_residual -ksp_converged_reason
-pc_fieldsplit_[0,1]_ksp_monitor_true_residual

3) We cannot say anything about performance without seeing the log for both
runs

  -log_view

   Thanks,

     Matt


> Best regards,
> Bernardo
>
>


-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
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