[petsc-users] [EXT]Re: MatZeroRows

[Anthony Paul Haas]

Awesome! thanks to all.

Cheers

On Thu, Aug 13, 2020 at 2:19 PM Barry Smith <bsmith at petsc.dev> wrote:

> *External Email*
>
>   Anthony,
>
>     Keeping the nonzero locations (with zero in them) will be a bit faster
> in the call to MatZeroRows() because otherwise it has to shift all the
> entries in the sparse matrix arrays data structures to "remove" the
> unneeded locations.
>
>     But the real question is how it affects times in later function calls
>
> 1)  If you do the MatSetValues()/MatAssemblyBegin/End() repeatedly it will
> be much faster if you keep the nonzero locations (with zero in them).
>
> 2)  Time for MatMult() will be a bit faster if you remove the locations,
> but the depending on the preconditioner the preconditioner may be more or
> less effective and a bit slower or faster.
>
>   So, rule of thumb is if you only do MatZeroRows() once you might
> generally remove the locations but if it is done in a loop with the same
> matrix (over time-steps for example, or even in SNES) you want to keep the
> nonzero locations.
>
>   Barry
>
> Note: in both cases the memory usage is the same because PETSc does not
> "return" the excessive memory.
>
>
> On Aug 13, 2020, at 8:07 AM, Matthew Knepley <knepley at gmail.com> wrote:
>
> On Thu, Aug 13, 2020 at 3:17 AM Anthony Paul Haas <aph at email.arizona.edu>
> wrote:
>
>> Hello,
>>
>> I am using MatZeroRows for imposing a known forcing into my equations in
>> conjunction with rhs and by setting the diagonal of the matrix to 1.
>>
>> I am using Fortran. I have used:
>>
>> ! only local processors set their own zeros
>>
>>
>> call MatSetOption(self%fieldLHSMat_ps, MAT_NO_OFF_PROC_ZERO_ROWS,
>> PETSC_TRUE, ierr)
>>
>> call MatSetOption(self%fieldLHSMat_ps, MAT_KEEP_NONZERO_PATTERN,
>> PETSC_TRUE, ierr)
>>
>> call MatZeroRows(self%fieldLHSMat_ps, numrows, glob_row_idx, diag,
>> PETSC_NULL_OBJECT, PETSC_NULL_OBJECT, ierr)
>>
>>
>> Is numrows above the local (on each proc.) number of rows to remove, or
>> is it the global number of rows to remove?
>>
>
> Local.
>
>
>> Also on some processors, I have no rows to remove, hence the array glob_row_idx
>> is not allocated. How can I tell Petsc? Should I pass PETSC_NULL_OBJECT
>> instead of glob_row_idx in this case?
>>
>
> If you pass numrows = 0, it should not matter what is in the next slot, as
> long as it type checks.
>
>
>> Finally, does using the option MAT_KEEP_NONZERO_PATTERN have an influence
>> on the time the MatZeroRows call will take?
>>
>
> I don't think it makes much of a difference, but I have not measured it.
>
>   Thanks,
>
>      Matt
>
>
>> Thanks,
>>
>> Best regards,
>>
>> Anthony
>>
>>
>>
>>
>
> --
> 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/
> <http://www.cse.buffalo.edu/~knepley/>
>
>
>
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