Petsc questions

[Barry Smith]

On Aug 14, 2008, at 1:33 PM, Matthew Knepley wrote:

> On Thu, Aug 14, 2008 at 1:22 PM, Nguyen, Hung V ERDC-ITL-MS
> <Hung.V.Nguyen at usace.army.mil> wrote:
>>
>> Barry,
>>
>> Thanks for the info. I will use what we already have. How's about  
>> estimation
>> of matrix condition number via PETSC?
>
> You can use -ksp_monitor_singular_value to print out the exterior
> singular values
> for the Hessenberg matrix generated by the Krylov method. I don't  
> think we have
> anything else. Condition number estimation is a tough problem.
>

    See also KSPSetComputeSingularValues(), KSPSetComputeEigenvalues(),
KSPComputeSingularValues(), KSPComputeEigenvalues() for use in your  
code.
I've found if the operator is not terribly ill-conditioned these can  
work fairly well.
If using GMRES for a non-symmetric matrix make sure you use a large  
enough
restart to get accurate values.

    Barry


>  Matt
>
>> -Hung
>>
>> -----Original Message-----
>> From: owner-petsc-users at mcs.anl.gov [mailto:owner-petsc-users at mcs.anl.gov 
>> ] On
>> Behalf Of Barry Smith
>> Sent: Thursday, August 14, 2008 11:08 AM
>> To: petsc-users at mcs.anl.gov
>> Subject: Re: Petsc questions
>>
>>
>>   Check AO functions. They renumber between two different orderings.
>>
>> But, if your code already puts everything in the new numbering, you  
>> can just
>> use PETSc with that new numbering and leave your ordering code as  
>> it is.
>>
>>   Barry
>>
>> On Aug 14, 2008, at 9:37 AM, Nguyen, Hung V ERDC-ITL-MS wrote:
>>
>>>
>>> Hello All,
>>>
>>> I am new to PETSC. I have following questions about PETSC:
>>>
>>> 1. Do PETSC functions help to estimate/calculate a matrix condition
>>> number?
>>> If yes, can I get the info how to do it?
>>>
>>> 2. A question about renumbering nodes: our CFD code uses ParMetis to
>>> compute the original partitioning of the mesh.  The global nodes are
>>> renumbered consecutively within each Parmetis partition as npetsc
>>> which is a mapping vector from the original global node numbering to
>>> the new numbering, see below as a test code. My question is whether
>>> PETSC function helps to renumber from ParMetis partition to PETSC
>>> partition or not?
>>>
>>> Thank for your help.
>>>
>>> Regards,
>>>
>>> -Hung
>>> -- code:
>>> !         Read the data.
>>>
>>>     fname = 'petsc.dat'
>>>     call parnam (fname)
>>>     open (2, file = fname, status = 'old')
>>>
>>> !         No. global nodes, local nodes, owned nodes,
>>> !         compressed columns, PEs.
>>>
>>>     read (2, '(5i10)') ng, nloc, nown, ncol, npes
>>>
>>>     if (noproc .ne. npes) then
>>>       if (myid .eq. 0) then
>>>         print*, 'Number of PEs from the data file does not match',
>>>    &      ' the number from the run command.'
>>>       end if
>>>       call PetscFinalize (ierr)
>>>       stop
>>>     end if
>>>
>>> !         Local node array containing global node numbers.
>>>
>>>     allocate (nglobal(nloc))
>>>
>>>     read (2, '(8i10)') nglobal
>>>
>>> !         Find petsc numbering scheme.
>>>
>>>     allocate (nown_all(noproc))
>>>     allocate (idisp(noproc))
>>>     allocate (npetsc(nloc))
>>>     allocate (nodes1(ng))
>>>     allocate (nodes2(ng))
>>>
>>>     call MPI_ALLGATHER (nown, 1, MPI_INTEGER, nown_all, 1,
>>>    &  MPI_INTEGER, PETSC_COMM_WORLD, ierr)
>>>
>>>     idisp(1) = 0
>>>     do i = 2, noproc
>>>       idisp(i) = idisp(i - 1) + nown_all(i - 1)
>>>     end do
>>>
>>>     call MPI_ALLGATHERV (nglobal, nown, MPI_INTEGER, nodes1,
>>>    &   nown_all, idisp, MPI_INTEGER, PETSC_COMM_WORLD, ierr)
>>>
>>>     do i = 1, ng
>>>       ii = nodes1(i)
>>>       nodes2(ii) = i
>>>     end do
>>>
>>> !         Process the local nodes for their petsc numbers.
>>>
>>>     do i = 1, nloc
>>>       ii = nglobal(i)
>>>       npetsc(i) = nodes2(ii)
>>>     end do
>>>
>>>
>>
>>
>
>
>
> -- 
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which
> their experiments lead.
> -- Norbert Wiener
>