[petsc-users] zero pattern of result of matmatmult

[Jed Brown]

Frederik Treue <frtr at fysik.dtu.dk> writes:

> On Tue, 2013-09-17 at 08:15 -0500, Barry Smith wrote:
>>     Are you calling MatSetValuesStencil() for each single entry or once per row or block row? Row or block row will be faster than for each entry.
> once per entry - this I should definitely improve. What's the logic
> behind row or block rows being faster? The fewer calls to
> MatSetValuesStencil, the better?

It shares index translation and the call stack.

>> 
>>     Since you are using MatSetValuesStencil() I assume you are using a DA?
>
> I am, with one DOF per point. Are there better alternatives?

No, what you're doing is fine.

> PS. FYI: My stencil is essentially a standard 2D, 9-point box stencil,
> but the values in all rows are different (due, among other things, to
> the geometric tensor).

That's normal.  Is the geometry changing?

>       ##########################################################
>       #                                                        #
>       #                          WARNING!!!                    #
>       #                                                        #
>       #   This code was compiled with a debugging option,      #
>       #   To get timing results run ./configure                #
>       #   using --with-debugging=no, the performance will      #
>       #   be generally two or three times faster.              #
>       #                                                        #
>       ##########################################################

We weren't joking.

You should configure an optimized build using a different value of
PETSC_ARCH so that you can easily switch back and forth between the
debug and optimized versions.  Everything will get faster, but
especially insertion.
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