[petsc-users] PETSc KSPBCGSL with HYPRE boomeramg preconditioner ksp_monitor in poisson function

[Hao Zhang]

hi,

I am using KSPBCGSL method and HYPRE boomeramg precondtioner passed into
KSPsolve for poisson equation as part of incompressible NS.

I have two questions
Q1 is number of iterations for the same solver. max_iter = 20 for the same
poisson function

KSPGetIterationNumber() is used to set up max_iter = 20


 0 KSP Residual norm 4.512245447770e-04
  2 KSP Residual norm 6.396709069731e-07
  4 KSP Residual norm 1.757784220489e-10
  6 KSP Residual norm 1.501464709144e-14
  8 KSP Residual norm 1.376381429122e-18
 10 KSP Residual norm 1.072650548945e-19
In poisson_func(): num_iter = 10, rel_residual = 1.072651e-19

and the very same function at the next time step gives: remind you guys
max_iter = 20. I assume the number of iterations should stop at num_iter =
2. but the solver seems to keep iterating util hitting the max_iter.
 0 KSP Residual norm 5.107005838093e-16
  2 KSP Residual norm 3.353634821198e-20
  4 KSP Residual norm 1.835096039266e-23
  6 KSP Residual norm 1.645496102409e-23
  8 KSP Residual norm 1.645496099837e-23
 10 KSP Residual norm 1.645496099836e-23
 12 KSP Residual norm 1.645496099530e-23
 14 KSP Residual norm 1.645496034473e-23
 16 KSP Residual norm 1.645461130961e-23
 18 KSP Residual norm 1.645460451075e-23
In poisson_func(): num_iter = 18, rel_residual = 1.645460e-23

Q2 is: it seems HYPRE with KSPsolve works with coarse mesh but when I
double the mesh size. the solve halted indefinitely. ksp_monitor didn't
produce any results.
is there a way to understand what's happening?

-- 
Hao Zhang
Dept. of Applid Mathematics and Statistics,
Stony Brook University,
Stony Brook, New York, 11790
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