[petsc-users] Some help for partitioning a vector

[Barry Smith]

  Only parallel vectors can be multiplied with parallel matrices and the vector and matrix must exist on the same MPI communicator. In your case you are trying to multiple a sequential vector by a parallel matrix.  You should use VecCreateMPI(PETSC_COMM_WORLD, PETSC_DECIDE, size_,&vol_);
                                                                                                                                         ^^^^^^ 
                      use PETSC_COMM_WORLD or the communicator you used in creating the matrix, don't use PETSC_COMM_SELF


> On Jun 28, 2016, at 9:14 AM, ehsan sadrfaridpour <it.sadr at gmail.com> wrote:
> 
> Hi, 
> Sorry for long email. My questions are in the bottom.
> 
> I try to use MatMult to multiply a vector to a matrix but I face below problem. 
> [0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
> [0]PETSC ERROR: Nonconforming object sizes
> [0]PETSC ERROR: Incompatible partition of A (20) and xx (40)
> [0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for trouble shooting.
> [1]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
> [1]PETSC ERROR: Nonconforming object sizes
> [1]PETSC ERROR: Incompatible partition of A (20) and xx (40)
> [1]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html for trouble shooting.
> [1]PETSC ERROR: Petsc Release Version 3.6.3, unknown 
> 
> I use 2 process for this. I check the local ranges with VecGetOwnershipRange and MatGetOwnershipRange commands and they reveal that the matrix is distributed between processes while the vector is exist completely on both processes.
> in rank:1, vector start:0, end:40, matrix start:20, end:40
> in rank:0, vector start:0, end:40, matrix start:0, end:20
> 
> I created the vector "vol_" using below commands.
>    Vec vol_;
>     VecCreateMPI(PETSC_COMM_SELF, PETSC_DECIDE, size_,&vol_);
>     VecSet(vol_,val);           //Sets all components of a vector to a single scalar value.
>     VecAssemblyBegin(vol_);
>     VecAssemblyEnd(vol_);
> 
> My questions:
> A- What should I do differently to create a scattered vector in the first place?
> B- If my vector initialization is ok, what should I do now to scatter the vector among the processes? 
> I saw the VecScatter function, but I am not sure how to use it, if it is the right function.
> 
> Best,
> Ehsan
>