[petsc-users] MatChop

[Lawrence Mitchell]

> On 21 May 2021, at 17:53, Stefano Zampini <stefano.zampini at gmail.com> wrote:
> 
>> I see, anyway you do not need the check if the loop range [rStart,rEnd). So now I don’t understand why the loop must be [rStart,rStart+maxRows], Matt?
>> 
>> It is terrible, but I could not see a way around it. We want to use MatGetRow() for each row, but that requires an assembled matrix.
> 
> What is the use case for calling MatChop on an unassembled matrix ?

It's rather that the matrix is modified "through the front door" by just calling MatSetValues repeatedly to replace the small values with zero. This is because an in-place modification of the matrix would require a method for each matrix type.

So the process is:

for each row:
    rowvals = MatGetRow(row)
    rowvals[abs(rowvals) < tol] = 0
    MatSetValues(rowvals, ..., INSERT)
    MatRestoreRow(row)
    MatAssemblyBegin/End <- so that the next MatGetRow does not error.

Now, one "knows" that this assembly will not need to communicate (because you only set local values), but the automatic state tracking can't know this.

A disgusting hack that is tremendously fragile would be to do:

for each row:
    ...
    mat->assembled = PETSC_TRUE
mat->assembled = PETSC_FALSE
MatAssemblyBegin/End

But I would probably refuse to accept that could :)

Lawrence