[petsc-users] Fwd: SNES with Fieldsplit + MatNest in parallel

[Nicolas Barnafi]

Thanks for the help Matthew.
>
> The next step would be to verify that the operators you get in those 
> slots are correct in parallel. I normally just print out the matrix
> for a small problem. This is the only way I know to debug assembly.
>
> To check the solver, you set it to an exact solver and see that it 
> takes 1 iterate. That means using LU as the subsolver
> for the first block, using the full Schur complement, and solving the 
> Schur system with a residual tolerance of something like 1e-10.
>

There is certainly something wrong in the nested matrix, as this is the 
output in serial (LU + full Schur LU):

   0 SNES Function norm 3.240309155660e+03
     0 KSP Residual norm 3.240309155660e+03
     1 KSP Residual norm 3.219893365194e-08
   1 SNES Function norm 3.999872813081e+03
     0 KSP Residual norm 1.214145680994e+04
     1 KSP Residual norm 2.742817643080e-04
   2 SNES Function norm 3.663148395735e+01
     0 KSP Residual norm 3.980103473350e+03
     1 KSP Residual norm 1.814754406828e-05
   3 SNES Function norm 2.136252100504e+01
     0 KSP Residual norm 6.970609974245e+01
     1 KSP Residual norm 1.982821753925e-05
   4 SNES Function norm 1.059694009734e-02
     0 KSP Residual norm 2.136131725219e+01
     1 KSP Residual norm 4.545878835451e-06
     2 KSP Residual norm 8.841427426699e-08
     3 KSP Residual norm 3.305898584949e-09
   5 SNES Function norm 3.729120062850e-03
     0 KSP Residual norm 3.846880232871e-03
     1 KSP Residual norm 1.740268533385e-06
     2 KSP Residual norm 3.013108797112e-08
     3 KSP Residual norm 1.105602265959e-09
   6 SNES Function norm 1.117697448247e-09

whereas in parallel it is this (two processes):

   0 SNES Function norm 3.240309155660e+03
     0 KSP Residual norm 3.240309155660e+03
     1 KSP Residual norm 2.158162432432e-01
     2 KSP Residual norm 2.156121452805e-01
     3 KSP Residual norm 2.148639482595e-01
     4 KSP Residual norm 2.148248193325e-01
     5 KSP Residual norm 1.305140166188e-03
   Linear solve converged due to CONVERGED_RTOL iterations 5
   1 SNES Function norm 4.774611203034e+42


Best,
NB
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20211109/57238a88/attachment.html>