[petsc-users] MatSetValues

[Vasileios Kalantzis]

Hi Barry,

yes, I am inserting new locations. I was actually copying matrix
C to a new matrix D, and then I was inserting the "thresholded"
values -E*(B\E) in this matrix D.

I was pretty much sure that the missing pre-allocation was the
reason for the slow code -- when I commented out the MatSetValues()
part (i.e. I approximated the Schur complement only by matrix C),
this part was much much faster.

I will definitely follow your suggestion -- Thanks!!!


On Mon, Jan 25, 2016 at 1:07 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Jan 25, 2016, at 12:15 PM, Vasileios Kalantzis <kalan019 at umn.edu>
> wrote:
> >
> > Dear all,
> >
> > I am trying to form an approximation of the
> > Schur complement S = C-E'*(B\E) of a matrix
> > A = [B, E ; E', C]. Matrix C is stored as a
> > distributed Mat object while matrices B and E
> > are locally distributed to each processor (the
> > block partitioning comes from a Domain
> > Decomposition point-of-view). All matrices B, E,
> > and C are sparse.
> >
> > I already have a sparse version of -E'*(B\E)
> > computed. Moreover, -E'*(B\E)  is block-diagonal.
> > The only issue now is how to merge (add) C and
> > -E'*(B\E). The way i do the addition right now is
> > based on checking every entry of -E'*(B\E) and,
> > if larger than a threshold value, add it to C using
> > the MatSetValue routine. The above is being done
> > in parallel for each diagonal block of -E'*(B\E).
> >
> > The code works fine numerically but my approach
> > is too slow if -E'*(B\E) is not highly sparse.
>
>    This is a guess, but if you inserting new nonzero locations into C with
> MatSetValues() then this will be very slow. Are you inserting new locations?
>
>    If so, here is what you need to do. Sweep through the rows of
> C/-E'*(B\E)  determining the number of nonzeros that will be in the result
> and then use MatCreateMPIAIJ() or MatMPIAIJSetPreallocation() to
> preallocate the space in a new matrix, say D. Then sweep through all the
> rows again actually calling the MatSetValues() and put the entries into D.
> Switching from non-preallocation to preallocation will speed it up
> dramatically (factors of 100's or more) if you were inserting new locations.
>
>    Barry
>
>
>
> >
> > I know that I can set many entries together by
> > using the MatSetValues routine but I am not
> > sure how to do it because the sparsity pattern of
> > each column of -E'*(B\E) differs. Maybe I can
> > assemble the sparsified Schur complement
> > column-by-column using MatSetValues but is
> > there any other idea perhaps?
> >
> > Thanks ! :)
>
>
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