[petsc-users] about MatTranspose

[Jed Brown]

On Thu, Sep 1, 2011 at 10:46, Likun Tan <likunt at andrew.cmu.edu> wrote:

> I tried to use
> PetscSplitOwnership(PETSC_COMM_WORLD,&n,&N);
> MatCreateSeqDense(PETSC_COMM_SELF,M,n,PETSC_NULL,&B);
>
> But it seems not working. This command is make B store in N/n processors
> with n columns in each processor?
>

B (in the code) is a different matrix on each process. You should fill the
columns so that each process has part of B (the logical thing you are
working with).


> I try to calculate the N/n parts in
> parallel, but when i apply MatGetOwnershipRange(), i found the matrix will
> be automatically partitioned by row.
>

MatGetOwnershipRange() just gives you the rows from 0 to 26. You need to sum
the column partition that was computed earlier. The most explicit way to do
that is

PetscInt cstart;
MPI_Scan(&n,&cstart,1,MPIU_INT,MPI_SUM,PETSC_COMM_WORLD);
cstart -= n;

Now each process should compute columns of the logical B in the range
[cstart,cstart+n). The B object in your code will indexed [0,n) on each
process, but these columns correspond to [cstart,cstart+n).


> That's why i go back to use MatCreate() to create a 200*27 matrix and try
> to get the tranpose of it.I used
> MatCreateSeqDense(PETSC_COMM_SELF, M, N, PETSC_NULL, &C)
> to create C, there is no partition of C, i guess this is the reason of the
> error.
>

Don't go down this route, it doesn't do what you want.
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