[petsc-users] BJACOBI with FIELDSPLIT

[Matthew Knepley]

On Mon, Mar 18, 2019 at 3:18 PM Rossi, Simone via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Dear all,
>
> I'm debugging my application in which I'm trying to use the FIELDSPLIT
> preconditioner for solving a 2x2 block matrix.
>
>
> Currently I'm testing the preconditioner on a decoupled system where I
> solve two identical and independent Poisson problems. Using the default
> fieldsplit type (multiplicative), I'm expecting the method to be equivalent
> to a Block Jacobi solver.
>
> Setting
>
> -ksp_rtol 1e-6
>
> while  using gmres/hypre on each subblock with
>
> -fieldsplit_0_ksp_rtol 1e-12
>
> -fieldsplit_1_ksp_rtol 1e-12
>
> I'm expecting to converge in 1 iteration with a single solve for each
> block.
>
>
> Asking to output the iteration count for the subblocks with
>
> -ksp_converged_reason
> -fieldsplit_0_ksp_converged_reason
>
> -fieldsplit_1_ksp_converged_reason
>
> revealed that the outer solver converges in 1 iteration, but each block is
> solved for 3 times.
>
> This is the output I get:
>
>
>   Linear fieldsplit_0_ solve converged due to CONVERGED_RTOL iterations 7
>   Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 7
>

This first application of the PC is to evaluate the initial preconditioned
residual.


>   0 KSP preconditioned resid norm 9.334948012657e+01 true resid norm
> 1.280164130222e+02 ||r(i)||/||b|| 1.000000000000e+00
>
>   Linear fieldsplit_0_ solve converged due to CONVERGED_RTOL iterations 7
>   Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 7
>

This next one is for applying M^{-1} A in the Krylov iteration.


>   Linear fieldsplit_0_ solve converged due to CONVERGED_RTOL iterations 7
>   Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 7
>

I think this one might be from building the true residual.

  Matt


>   1 KSP preconditioned resid norm 1.518151977611e-11 true resid norm
> 8.123270435936e-12 ||r(i)||/||b|| 6.345491366429e-14
>
> Linear solve converged due to CONVERGED_RTOL iterations 1
>
>
>
> Are the subblocks actually solved for multiple times at every outer
> iteration?
>
>
> Thanks for the help,
>
>
> Simone
>
>
>
>

-- 
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.cse.buffalo.edu/~knepley/>
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