[petsc-users] Error with KSPSetUp and MatNest

Smith, Barry F. bsmith at mcs.anl.gov
Thu Apr 11 00:48:47 CDT 2019

  Here is my guess at what is going wrong (and if it is the case we should figure out how to fix it). The KSPSetUp() is triggering the creation of 
work vectors and the type of work vector created at this point is not compatible with the vectors passed into KSPSolve() thus generating an error.

  Meanwhile if you skip calling KSPSetUp(), the KSPSolve() calls KSPSetUp() and by that time KSP knows the vector type (based on the vector passed 
into KSPSolve()) and uses that as a template to generate the work vectors which are now compatible since they were generated using the input vector. It may require using the debugger to track why the wrong type of vector is created for work vectors in KSPSetUp(). It is too late at night for me to try to debug it now.


> On Apr 10, 2019, at 11:57 AM, Matthew Knepley via petsc-users <petsc-users at mcs.anl.gov> wrote:
> On Wed, Apr 10, 2019 at 12:49 PM Manuel Colera Rico <m.colera at upm.es> wrote:
> Thank you for your answer, Matt. In the MWE example attached before, both Nest vectors (the r.h.s. of the system and the vector of unknowns) are composed of the same number of blocks (2). Indeed, PETSc is able to solve the system if KSPSetUp() is not called, so the system/MatNest/MatVec's must not incompatible at all. Therefore, I wonder if I have missed to called something before this routine or if this is a KSPSetUp's bug.
> I will run it. 
> Of course one can always directly define a single matrix and a single vector, but I find it easier to work with Nest matrices and vectors. Moreover, I think that the moment to use them is from the beginning... once all the code is developed, it is very hard to switch matrices types.
> This is not the case, and its the major reason why I want to remove Vec/MatNest from the public interface. This is a major misconception. You can
> switch matrix types using -mat_type <type>. You can pull out submatrices using MatGetSubmatrix() from MatAIJ just as simply as MatNest.
> If you want to keep assembling submatrices, you can use 
>   https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatCreateLocalRef.html#MatCreateLocalRef
> so that the assembly looks just like it does now for you, but it assembles to an AIJ _or_ a Nest, depending on what you
> choose at the command line. Then you can debug everything as AIJ, and finally switch to Nest if you need it for optimization.
>   Thanks,
>      Matt
> Regards,
> Manuel
> ---
> On 4/10/19 5:41 PM, Matthew Knepley wrote:
>> On Wed, Apr 10, 2019 at 11:29 AM Manuel Colera Rico via petsc-users <petsc-users at mcs.anl.gov> wrote:
>> Hello,
>> I am trying to solve a system whose matrix is of type MatNest. If I 
>> don't use KSPSetUp(), everything is fine. However, if I use that 
>> routine, I get the following error:
>> 0]PETSC ERROR: --------------------- Error Message 
>> --------------------------------------------------------------
>> [0]PETSC ERROR: Invalid argument
>> [0]PETSC ERROR: Nest vector arguments 1 and 2 have different numbers of 
>> blocks.
>> This seems self-explanatory. Nest vectors must have the same number of blocks to be compatible.
>> More broadly, there should be no reason to use Nest vectors or matrices. It is an optimization to
>> be used at the very end, only after you have profiled the code and seen that its important. You can
>> do everything you want to do without ever touching Nest, and it looks like the Nest interface is a
>> problem for your code right now.
>>   Thanks,
>>     Matt
>> [0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html 
>> for trouble shooting.
>> [0]PETSC ERROR: Petsc Release Version 3.11.0, unknown
>> /home/manu/Documents/FEM-fluids/C-codes/CLG2-ConvectionDiffusion/Debug/CLG2-ConvectionDiffusion 
>> on a mcr_20190405 named mancolric by Unknown Wed Apr 10 17:20:16 2019
>> [0]PETSC ERROR: Configure options --with-cc=gcc --with-cxx=g++ 
>> --with-fc=gfortran COPTFLAGS="-O3 -march=native -mtune=native" 
>> CXXOPTFLAGS="-O3 -march=native -mtune=native" FOPTFLAGS="-O3 
>> -march=native -mtune=native" --with-debugging=0 --download-fblaslapack 
>> --download--f2cblaslapack --download-mpich --download--hypre 
>> --download-scalapack --download-mumps --download-suitesparse 
>> --download-ptscotch --download-pastix --with-matlab --with-openmp
>> [0]PETSC ERROR: #1 VecCopy_Nest() line 68 in 
>> /opt/PETSc_library/petsc-3.11.0/src/vec/vec/impls/nest/vecnest.c
>> [0]PETSC ERROR: #2 VecCopy() line 1614 in 
>> /opt/PETSc_library/petsc-3.11.0/src/vec/vec/interface/vector.c
>> [0]PETSC ERROR: #3 KSPInitialResidual() line 63 in 
>> /opt/PETSc_library/petsc-3.11.0/src/ksp/ksp/interface/itres.c
>> [0]PETSC ERROR: #4 KSPSolve_GMRES() line 236 in 
>> /opt/PETSc_library/petsc-3.11.0/src/ksp/ksp/impls/gmres/gmres.c
>> [0]PETSC ERROR: #5 KSPSolve() line 782 in 
>> /opt/PETSc_library/petsc-3.11.0/src/ksp/ksp/interface/itfunc.c
>> [0]PETSC ERROR: #6 mwe() line 55 in ../Tests/tests.c
>> Please find attached a MWE (it is a slight modification of that of the 
>> post opened by Ce Qin, 
>> https://lists.mcs.anl.gov/pipermail/petsc-users/2015-February/024230.html, 
>> whose answer I have not found).
>> By the way, with the newest version of PETSc, Eclipse marks as errors 
>> the commands PetscFree, CHKERRQ, PETSC_COMM_SELF,... although it 
>> compiles and executes well. Perhaps it is a problem related to Eclipse, 
>> but this did not happen with the older versions of PETSc.
>> Thanks and regards,
>> Manuel
>> ---
>> -- 
>> 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
>> https://www.cse.buffalo.edu/~knepley/
> -- 
> 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
> https://www.cse.buffalo.edu/~knepley/

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