[petsc-users] The same code randomly produce different residual norms

[Fande Kong]

Hi,

I run the code src/snes/examples/tutorials/ex5.c with options:











* mpirun  -n 8  ./ex5 -pc_type mg \ -ksp_monitor\ -pc_mg_levels 3
\ -pc_mg_galerkin \ -da_grid_x 17 \ -da_grid_y 17
\ -mg_levels_ksp_norm_type unpreconditioned \ -snes_monitor
\ -mg_levels_ksp_chebyshev_estimate_eigenvalues 0.5,1.1
\ -mg_levels_pc_type sor \ -pc_mg_type \*


I run this script several times, but at each time, the residual norms have
some tiny differences ( these should not happen). For example:

Case 1:
















*  0 SNES Function norm 1.188788066192e+00     0 KSP Residual norm
1.573384253521e+00     1 KSP Residual norm 3.616321708396e-02     2 KSP
Residual norm 2.780221563755e-04     3 KSP Residual norm 2.194662354037e-06
  1 SNES Function norm 5.125240595190e-03     0 KSP Residual norm
1.217284338266e-01     1 KSP Residual norm 1.774247017346e-04     2 KSP
Residual norm 2.557118591292e-06     3 KSP Residual norm 1.622173269367e-08
  2 SNES Function norm 3.922335995111e-05     0 KSP Residual norm
9.960745924876e-04     1 KSP Residual norm 1.273916336665e-06     2 KSP
Residual norm 1.571259383270e-08     3 KSP Residual norm 1.250266145356e-10
  3 SNES Function norm 2.662898279023e-09 *


Case 2:

















*  0 SNES Function norm 1.188788066192e+00     0 KSP Residual norm
1.573384253521e+00     1 KSP Residual norm 3.616321708396e-02     2 KSP
Residual norm 2.780221563755e-04     3 KSP Residual norm 2.194662354037e-06
  1 SNES Function norm 5.125240595190e-03     0 KSP Residual norm
1.217284338266e-01     1 KSP Residual norm 1.774247017347e-04     2 KSP
Residual norm 2.557118591292e-06     3 KSP Residual norm 1.622173269367e-08
  2 SNES Function norm 3.922335995108e-05     0 KSP Residual norm
9.960745924862e-04     1 KSP Residual norm 1.273916336654e-06     2 KSP
Residual norm 1.571259383257e-08     3 KSP Residual norm 1.250266145346e-10
  3 SNES Function norm 2.662898285038e-09 *

These differences are marked by red color. So here, I was wondering there
are any explanations why these happen?

Thanks,

Fande,
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