[petsc-users] ghost values

[Matthew Knepley]

On Tue, Apr 14, 2015 at 6:25 PM, Gideon Simpson <gideon.simpson at gmail.com>
wrote:

> So other than the minor point of it zeroing out the data, is there any
> conceivable reason to use Create/Destroy over Get/Restore?
>

Nope, and calling VecSet(v, 0.0) is really cheap.

  Matt


> -gideon
>
> On Apr 14, 2015, at 7:16 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
> On Tue, Apr 14, 2015 at 6:15 PM, Gideon Simpson <gideon.simpson at gmail.com>
> wrote:
>
>> Other than zeroing things out, is there any substantive difference
>> between DMCreateLocalVector and DMGetLocalVector?
>>
>
> The Get version caches vectors, so you are not continually
> creating/destroying
>
>    Matt
>
>
>> -gideon
>>
>> > On Apr 14, 2015, at 7:02 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>> >
>> >
>> >  When PETSc vectors are created initially they always have 0
>> everywhere. So if you use DMCreateLocalVector() it will have zero in all
>> those ghost places (as well as everywhere else).
>> >
>> >   But if you use DMGetLocalVector() it returns vectors that maybe dirty
>> so you need to fill in any locations you want to have a known value with
>> that known value. Or call VecSet() to clear the entire vector.
>> >
>> >  Barry
>> >
>> >> On Apr 14, 2015, at 5:52 PM, Gideon Simpson <gideon.simpson at gmail.com>
>> wrote:
>> >>
>> >> If i use the DM_BOUNDARY_GHOSTED flag in the creation of a DMDA array,
>> are the ghosted values automatically set to zero, or should they be
>> manually set to zero if that’s the desired ghost value?
>> >>
>> >> -gideon
>> >>
>> >
>>
>>
>
>
> --
> 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
>
>
>


-- 
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
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