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

[Barry Smith]

> On Sep 16, 2017, at 10:39 PM, Hao Zhang <hbcbh1999 at gmail.com> wrote:
> 
> 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

  What do you mean, KSPGetIterationNumber() tells you the current number of iterations. You cannot use it to set the maximum number of iterations. Use KSPSetTolerances() or -ksp_max_it <its> to set the number
> 
> 
>  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.

   Why would it stop at num_iter 2. The number of iterations allowed is the same for each solve, it does not accumulate over all solves.

>  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 

   But the way it is almost never realistic to try to get residual norms to be this small. You might consider using something like -ksp_atol 1.e-16 or something 

> 
> 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?

   You could run in the debugger and use control c to interrupt the code when it is "hanging" and type bt to see the stack trace for the hanging.


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