[petsc-users] Parallel Incomplete Choleski Factorization

[Barry Smith]

>> Please update to petsc-3.3. petsc-3.1 is too old.
>     I would do that but the version installed on the platform (Intrepid at ALCF) I am working on is 3.1-p2.

   Satish,

      Please fix this.

     Thanks

     Barry

On Jul 17, 2012, at 1:36 PM, Michele Rosso wrote:

> 
> On 07/17/2012 11:03 AM, Hong Zhang wrote:
>> Michele :
>> 
>> I have some problems with the block jacobi preconditioner.
>> I am solving a 3D  Poisson equation with periodic BCs, discretized by using finite differences (7-points stencil).
>> Thus the problem is singular and the nullspace has to be removed.
>> 
>> For  Poisson equations, multigrid precondition should be the method of
>> choice. 
> Thank you for the suggestion. I do not have any experience with multigrid, but I will try.
>> 
>> If I solve with the PCG method + JACOBI preconditioner the results are fine.
>> If I use PCG + Block Jacobi preconditioner + ICC on each block the results are fine on the majority of the processors,
>> but on few of them the error is very large.  
>>  
>> How do you know " few of them"? 
>  Basically the solution is not correct on some grid points, say 6 grid nodes out of 64^3. The 6 grid nodes with problems belongs to 2 of the 128 processors
> I am using.
>> 
>> Do you have any idea/suggestions on how to fix this problem? 
>> This is the fragment of code I am using ( petsc 3.1 and Fortran 90):
>>  
>> Please update to petsc-3.3. petsc-3.1 is too old.
>     I would do that but the version installed on the platform (Intrepid at ALCF) I am working on is 3.1-p2.
> 
>> 
>>     PetscErrorCode                petsc_err     
>>     Mat                                         A
>>     PC                                           pc, subpc
>>     KSP                                         ksp
>>     KSP                                         subksp(1)
>>     :
>>     :
>>     :
>>     call KSPCreate(PETSC_COMM_WORLD,ksp,petsc_err)
>>     call KSPSetOperators(ksp,A,A,DIFFERENT_NONZERO_PATTERN,petsc_err)
>>  
>> call KSPSetType(ksp,KSPCG, ) !the default type is gmres. I guess you want CG
>> 
>>     call KSPGetPC(ksp,pc,petsc_err)
>>     call PCSetType(pc,PCBJACOBI,petsc_err)
>> !    call KSPSetUp(ksp,petsc_err)    call this at the end
>>     
>>     ! KSP context for each single block
>>     call PCBJacobiGetSubKSP(pc,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,subksp(1),petsc_err) 
>>     call KSPGetPC(subksp(1),subpc,petsc_err)
>>     call PCSetType(subpc,PCICC,petsc_err)
>>  
>>     call KSPSetType(subksp(1),KSPPREONLY petsc_err)
>>  
>>     call KSPSetTolerances(subksp(1),tol ,PETSC_DEFAULT_DOUBLE_PRECISION,&
>>          & PETSC_DEFAULT_DOUBLE_PRECISION,PETSC_DEFAULT_INTEGER,petsc_err)
>> 
>>    ! Remove nullspace from the singular system (Check PETSC_NULL)
>>     call MatNullSpaceCreate(MPI_COMM_WORLD,PETSC_TRUE,0,PETSC_NULL,nullspace,petsc_err)
>>     call KSPSetNullSpace(ksp, nullspace, petsc_err)
>>     call MatNullSpaceRemove(nullspace, b, PETSC_NULL,petsc_err)    
>> 
>>     call KSPSolve(ksp,b,x,petsc_err)
>> 
>> I modified your code slightly. All these options can be provided at runtime:
>> '-ksp_type cg -pc_type bjacobi -sub_pc_type icc'
>> 
>> Hong
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> On 07/13/2012 12:14 PM, Hong Zhang wrote:
>>> Michele :
>>> 
>>> I need to use the ICC factorization as preconditioner, but I noticed that no parallel version is supported.
>>> Is that correct?
>>> Correct.
>>>  
>>> If so, is there a work around, like building  the preconditioner "by hand" by using PETSc functions?
>>> You may try block jacobi with icc in the blocks  '-ksp_type cg -pc_type bjacobi -sub_pc_type icc'
>>> 
>>> Hong 
>>> 
>> 
>> 
>> 
> 
>