[petsc-users] Prometheus vs GAMG for elasticity/plasticity problems

[Jed Brown]

Thomas Gross <tgross at ilsb.tuwien.ac.at> writes:

> I ran it with:
> -ksp_type cg -pc_type gamg -pc_gamg_type agg -log_summary -ksp_monitor -ksp_view -options_left -mg_levels_ksp_max_it 1 > GAMG_log_max_it.txt
> and
> -ksp_type cg -pc_type gamg -pc_gamg_type agg -log_summary -ksp_monitor -ksp_view -options_left -mg_levels_ksp_type richardson -mg_levels_pc_type sor > GAMG_log_Rich.txt
> (Log files attached)
>
> However, the solution time did not decrease and I still needed to call KSPSolve 3 times although I had -mg_levels_ksp_max_it 1. 

Your original Prometheus output had

  KSPSolve               1 1.0 4.6679e-02 1.0 4.10e+07 1.2 3.0e+02 2.7e+03 1.9e+02 34100 99100 86  34100 99100 87  1631

Now you have

Cheby(2)/Jacobi

  KSPSolve               3 1.0 1.3235e-01 1.0 3.36e+07 1.0 8.5e+02 2.3e+03 1.2e+03 36100 99100 96  36100 99100 96   507

Cheby(1)/Jacobi

  KSPSolve               3 1.0 1.3466e-01 1.0 3.28e+07 1.0 8.5e+02 2.3e+03 1.2e+03 49100 99100 96  49100 99100 96   486

Richardson/SOR

  KSPSolve               3 1.0 1.2476e-01 1.0 3.36e+07 1.0 7.0e+02 2.4e+03 1.2e+03 38100 99100 96  38100 99100 96   538


The cost _per solve_ looks fine here, so why do you/FEAP call KSPSolve
three times when using GAMG, going to a residual norm of 1e-16 when you
were only going to rtol 1e-8 with Prometheus?  For example, here is the
convergence the With Prometheus:

    0 KSP Residual norm 1.444126847260e-01 
    1 KSP Residual norm 5.357525404213e-03 
    2 KSP Residual norm 1.471040678379e-03 
    3 KSP Residual norm 3.704652302293e-04 
    4 KSP Residual norm 9.809180893460e-05 
    5 KSP Residual norm 3.175497350277e-05 
    6 KSP Residual norm 8.859979496890e-06 
    7 KSP Residual norm 2.071384344082e-06 
    8 KSP Residual norm 5.035483717523e-07 
    9 KSP Residual norm 1.516500637412e-07 
   10 KSP Residual norm 5.134577847338e-08 
   11 KSP Residual norm 1.270806138401e-08 
   12 KSP Residual norm 3.074793756862e-09 
   13 KSP Residual norm 1.048189477307e-09 

and here is with GAMG Richardson/SOR:

    0 KSP Residual norm 1.407866441446e-01 
    1 KSP Residual norm 6.331036946837e-03 
    2 KSP Residual norm 2.744397946950e-04 
    3 KSP Residual norm 1.419791977305e-05 
    4 KSP Residual norm 7.670137315396e-07 
    5 KSP Residual norm 4.636628100571e-08 
    6 KSP Residual norm 3.374396803942e-09 
    7 KSP Residual norm 1.936237763969e-10 

The latter has reduced the residual more (in preconditioned norm) in
half the iterations.  Try running with -ksp_norm_type unpreconditioned
if you're concerned about the accuracy of the GAMG solve.
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