[petsc-users] Poor FAS convergence for linear problems when not monitoring on levels

[Lawrence Mitchell]

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Hi all,

I'm observing poor convergence of FAS on linear problems when I use a
snes_type of ksponly on each level, unless I also monitor the snes on
those levels.  This is reproducible with snes ex35.c:

Using one iteration of newton on each level:

$ ./ex35 -da_refine 2 -snes_type fas -snes_monitor_short
    -fas_coarse_snes_type newtonls
    -fas_coarse_ksp_type preonly
    -fas_coarse_pc_type lu
    -fas_levels_snes_type newtonls
    -fas_levels_snes_max_it 1
    -fas_levels_ksp_max_it 2
    -fas_levels_ksp_convergence_test skip

  0 SNES Function norm 7.46324
  1 SNES Function norm 0.0783234
  2 SNES Function norm 0.000381979
  3 SNES Function norm 2.81934e-06
  4 SNES Function norm 3.33505e-08

Using ksponly as the level snes type:

$ ./ex35 -da_refine 2 -snes_type fas -snes_monitor_short
    -fas_coarse_snes_type newtonls
    -fas_coarse_ksp_type preonly
    -fas_coarse_pc_type lu
    -fas_levels_snes_type ksponly
    -fas_levels_snes_max_it 1
    -fas_levels_ksp_max_it 2
    -fas_levels_ksp_convergence_test skip

  0 SNES Function norm 7.46324
  1 SNES Function norm 11.0718
  2 SNES Function norm 2.31708
  3 SNES Function norm 0.354363
  4 SNES Function norm 0.0966945
  5 SNES Function norm 0.0184193
  6 SNES Function norm 0.00553901
  7 SNES Function norm 0.000916548
  8 SNES Function norm 0.000235095
  9 SNES Function norm 5.78095e-05
 10 SNES Function norm 1.68048e-05
 11 SNES Function norm 3.03099e-06
 12 SNES Function norm 7.5299e-07
 13 SNES Function norm 2.18443e-07
 14 SNES Function norm 5.8899e-08

Same options, but turning on the monitors on each snes level:

$ ./ex35 -da_refine 2 -snes_type fas -snes_monitor_short
    -fas_coarse_snes_type newtonls
    -fas_coarse_ksp_type preonly
    -fas_coarse_pc_type lu
    -fas_levels_snes_type ksponly
    -fas_levels_snes_max_it 1
    -fas_levels_ksp_max_it 2
    -fas_levels_ksp_convergence_test skip
    -fas_levels_snes_monitor /dev/null

  0 SNES Function norm 7.46324
  1 SNES Function norm 0.0783234
  2 SNES Function norm 0.000381979
  3 SNES Function norm 2.81934e-06
  4 SNES Function norm 3.33505e-08


Any ideas?

Cheers,

Lawrence
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