[petsc-users] early convergence failure

[Barry Smith]

On Dec 11, 2012, at 10:00 AM, Daniel Arndt <daniel.arndt at stud.uni-goettingen.de> wrote:

> Thank you Barry for your suggestions.
> 
> The error I get is now KSP_DIVERGED_INDEFINITE_PC. The matrix that I try to invert is actually symmetric and positive definite. I was not aware that this can lead to a indefinite preconditioner.

   Absolutely. Many preconditioners do not retain this feature even in exact precision and with numerical effects it can even appear unexpected.

   By default BoomAMG doesn't retain this.

> If I use a Jacobi preconditioner or tell BoomerAMG that the matrix is symmetric I don't encounter any errors. So I'm quite for now :-)
> 
> Daniel
>>   Daniel,
>> 
>>      That message is not coming from PETSc so likely Deal.II is processing the result from KSPConvergedReason() and generating that less then totally useful output. If you run with -ksp_converged_reason PETSC will (if Deal.II processes PETSc options correctly) print a more complete reason.
>> 
>>     Off hand I am guessing that CG detected a non-symmetric or indefinite matrix or preconditioner which it cannot handle so it barfed out. You can run with GMRES instead of CG and if that converges then this is the likely explanation.
>> 
>>     Barry
>> 
>> 
>> On Dec 11, 2012, at 8:19 AM, Daniel Arndt <
>> daniel.arndt at stud.uni-goettingen.de
>> > wrote:
>> 
>> >
>>  Hello everyone,
>> 
>> >
>>  
>> 
>> >
>>  at the moment I'm trying to solve a Poisson problem with SIPG
>> 
>> >
>>  stabilization and discontinuous finite elements. The matrix is
>> 
>> >
>>  constructed in deal.II. When I try to solve this problem with PETSc's CG
>> 
>> >
>>  solver and a BlockJacobi preconditioner or a BoomerAMG preconditioner
>> 
>> >
>>  from the Hypre package I get this weird error message.
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>> >
>>  
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>>  Exception on processing:
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>> >
>>  Iterative method reported convergence failure in step 3 with residual
>> 
>> >
>>  1.50616
>> 
>> >
>>  Aborting!
>> 
>> >
>>  
>> 
>> >
>>  Since the solver is allowed to take 5000 steps this convergence failure
>> 
>> >
>>  is clearly early. Did anyone encounter such an error before? What can
>> 
>> >
>>  produce such an early convergence failure?
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>> >
>>  
>> 
>> >
>>  Thanks in advance,
>> 
>> >
>>  Daniel
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