[petsc-users] Parallel Incomplete Choleski Factorization

[Michele Rosso]

Thank a lot.

Please let me know when version 3.3 is available.

Michele

On 07/17/2012 12:13 PM, Barry Smith wrote:
>>> 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
>>>>
>>>
>>>
>>
>


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