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

[Barry Smith]

> On Sep 16, 2017, at 11:33 PM, Hao Zhang <hbcbh1999 at gmail.com> wrote:
> 
> thanks.
> 
> Yes. KSPSetTolerances is used.
> 
> tolerance is set to be 1.e-14

   There is the rtol and atol are you sure you set the atol to 1e-14?

> 
> I was thinking if there is a debug option I could use from PETSc to get some insight?  GDB was used.

   You can use the command line option -start_in_debugger 


> 
> On Sun, Sep 17, 2017 at 12:25 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
> > 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
> 
> 
> 
> 
> -- 
> Hao Zhang
> Dept. of Applid Mathematics and Statistics,
> Stony Brook University, 
> Stony Brook, New York, 11790