MatMult_Scatter woes

[Barry Smith]

On Apr 8, 2009, at 3:11 AM, Jed Brown wrote:

> On Tue 2009-04-07 21:02, Barry Smith wrote:
>>
>>   Jed,
>>
>>    Please remind me of what the simplest fix is? Can we just zero the
>> vector before the scatter for the MatMult and MatMultTranspose?
>> What about for MatMultAdd and MatMultTransposeAdd, does anything  
>> need to
>> be done?
>
> The simplest fix is to zero the vector before and always use  
> ADD_VALUES.

     Why ADD_VALUES? I don't see that as being any more "correct" then  
INSERT_VALUES
in the case when two different x locations are assigned to a single y  
location.

    Perhaps the matrix class based on the scatter should have an  
option allowing the
user to determine add or insert? MatScatterSetType()?

    Barry



>
> I pushed this a week ago and cited this thread in the commit message,
> see 6d3d60abcf7d.  It's just worth remembering that it can be  
> optimized
> if someone ever finds that this is a bottleneck.
>
> Jed