[petsc-users] Understanding inner vs outer fieldsplit convergence

[Barry Smith]

   David, with Schur complement preconditioning the nesting can be rather complicated and hard to track. We need to know exactly what monitors you have turned on and the output from -ksp_view in order to understand why you are seeing this (seemingly) strange effect. Send all command line arguments and if you are running a PETSc example.



> On Jan 26, 2017, at 1:26 PM, David Knezevic <david.knezevic at akselos.com> wrote:
> 
> I'm exploring fieldsplit with Schur (this continues from some emails I sent a few weeks ago about this topic), and I had a quick question about the inner vs outer convergence.
> 
> I've pasted the output below from "-ksp_monitor -fieldsplit_FE_split_ksp_monitor", and I'm just wondering about why the second outer iteration has two inner iteration loops, whereas all the other outer iterations have one inner iteration loop? I assume it is something to do with a convergence tolerance, but it's not clear to me which tolerance would control that.
> 
> Thanks,
> David
> 
> ------------------------------------------------------------------
> 
>     Residual norms for fieldsplit_FE_split_ solve.
>     0 KSP Residual norm 4.742303891408e+01 
>     1 KSP Residual norm 2.909253505630e-01 
>     2 KSP Residual norm 9.891933795059e-02 
>     3 KSP Residual norm 7.147789520745e-02 
>     4 KSP Residual norm 1.668752967907e-02 
>     5 KSP Residual norm 5.019869896662e-03 
>     6 KSP Residual norm 2.848579237244e-03 
>     7 KSP Residual norm 2.847897269641e-03 
>     8 KSP Residual norm 2.840502392022e-03 
>     9 KSP Residual norm 2.831875522381e-03 
>    10 KSP Residual norm 2.688309287993e-03 
>    11 KSP Residual norm 1.351494303229e-03 
>    12 KSP Residual norm 1.350874246297e-03 
>    13 KSP Residual norm 9.154691604943e-06 
>   0 KSP Residual norm 2.254632353893e+02 
>     Residual norms for fieldsplit_FE_split_ solve.
>     0 KSP Residual norm 4.742303891408e+01 
>     1 KSP Residual norm 2.909253505630e-01 
>     2 KSP Residual norm 9.891933795059e-02 
>     3 KSP Residual norm 7.147789520745e-02 
>     4 KSP Residual norm 1.668752967907e-02 
>     5 KSP Residual norm 5.019869896662e-03 
>     6 KSP Residual norm 2.848579237244e-03 
>     7 KSP Residual norm 2.847897269641e-03 
>     8 KSP Residual norm 2.840502392022e-03 
>     9 KSP Residual norm 2.831875522381e-03 
>    10 KSP Residual norm 2.688309287993e-03 
>    11 KSP Residual norm 1.351494303229e-03 
>    12 KSP Residual norm 1.350874246297e-03 
>    13 KSP Residual norm 9.154691604943e-06 
>     Residual norms for fieldsplit_FE_split_ solve.
>     0 KSP Residual norm 1.554697370480e-05 
>     1 KSP Residual norm 1.554471967929e-05 
>     2 KSP Residual norm 1.551293889691e-05 
>     3 KSP Residual norm 8.031337431574e-06 
>     4 KSP Residual norm 4.137185786243e-06 
>     5 KSP Residual norm 4.066606123330e-06 
>     6 KSP Residual norm 4.051107282928e-06 
>     7 KSP Residual norm 4.047442850256e-06 
>     8 KSP Residual norm 4.047129984657e-06 
>     9 KSP Residual norm 4.030697964677e-06 
>    10 KSP Residual norm 2.882383190940e-06 
>    11 KSP Residual norm 3.325005138484e-07 
>    12 KSP Residual norm 2.107354774516e-07 
>    13 KSP Residual norm 2.107005548204e-07 
>    14 KSP Residual norm 4.399320792736e-08 
>    15 KSP Residual norm 4.236902403786e-08 
>    16 KSP Residual norm 2.932877082709e-08 
>    17 KSP Residual norm 3.881909203171e-09 
>    18 KSP Residual norm 1.107791399514e-09 
>    19 KSP Residual norm 2.645048006100e-11 
>   1 KSP Residual norm 8.266776463696e-01 
>     Residual norms for fieldsplit_FE_split_ solve.
>     0 KSP Residual norm 9.262528453386e-08 
>     1 KSP Residual norm 5.683232925010e-10 
>     2 KSP Residual norm 1.915223168286e-10 
>     3 KSP Residual norm 1.397893184942e-10 
>     4 KSP Residual norm 1.691441435404e-11 
>     5 KSP Residual norm 6.138315243419e-12 
>     6 KSP Residual norm 5.576043830003e-12 
>     7 KSP Residual norm 5.574440028225e-12 
>     8 KSP Residual norm 5.559544964428e-12 
>     9 KSP Residual norm 5.539862581746e-12 
>    10 KSP Residual norm 5.258329460152e-12 
>    11 KSP Residual norm 2.643581511791e-12 
>    12 KSP Residual norm 2.641293392449e-12 
>    13 KSP Residual norm 2.354608977643e-14 
>   2 KSP Residual norm 4.450925351013e-07 
>     Residual norms for fieldsplit_FE_split_ solve.
>     0 KSP Residual norm 6.653681330477e-14 
>     1 KSP Residual norm 6.650750698147e-14 
>     2 KSP Residual norm 6.111123464526e-14 
>     3 KSP Residual norm 2.026817941567e-14 
>     4 KSP Residual norm 9.604999144183e-15 
>     5 KSP Residual norm 9.208296307424e-15 
>     6 KSP Residual norm 9.196769686859e-15 
>     7 KSP Residual norm 9.185058975459e-15 
>     8 KSP Residual norm 9.180207477303e-15 
>     9 KSP Residual norm 8.991574890909e-15 
>    10 KSP Residual norm 8.032736869820e-15 
>    11 KSP Residual norm 1.536409278928e-15 
>    12 KSP Residual norm 1.177374264280e-15 
>    13 KSP Residual norm 1.175712092044e-15 
>    14 KSP Residual norm 2.572275406087e-16 
>    15 KSP Residual norm 2.548423809711e-16 
>    16 KSP Residual norm 8.616505207588e-17 
>    17 KSP Residual norm 7.563053994201e-18 
>    18 KSP Residual norm 6.807636198601e-18 
>    19 KSP Residual norm 9.747028518744e-19 
>    20 KSP Residual norm 2.419807103570e-21 
>   3 KSP Residual norm 2.986369469883e-09 
>     Residual norms for fieldsplit_FE_split_ solve.
>     0 KSP Residual norm 7.813223137340e-16 
>     1 KSP Residual norm 4.793103235095e-18 
>     2 KSP Residual norm 1.615526128222e-18 
>     3 KSP Residual norm 1.179102504397e-18 
>     4 KSP Residual norm 1.427467627551e-19 
>     5 KSP Residual norm 5.177440470993e-20 
>     6 KSP Residual norm 4.703763659148e-20 
>     7 KSP Residual norm 4.701953228322e-20 
>     8 KSP Residual norm 4.689269668869e-20 
>     9 KSP Residual norm 4.672625361251e-20 
>    10 KSP Residual norm 4.435174006113e-20 
>    11 KSP Residual norm 2.229156843383e-20 
>    12 KSP Residual norm 2.228887211080e-20 
>    13 KSP Residual norm 3.492936921635e-22 
>   4 KSP Residual norm 3.753341263086e-15