[petsc-users] Shell matrix and MatMultAdd.

[Vijay S. Mahadevan]

>   I suggest making a shell MatMultAdd_Mine() that does everything then
> also code a MatMult_Mine() that zeros the output vector and then calls directly MatMultAdd_Mine(). This way you'll
> have one code to routine but can handle both operations.

Barry, thanks for the suggestion. I will implement the MatMultAdd in
this way. Puristically speaking though, this does reverse the
dependency between the two routines !

Vijay

On Thu, Dec 2, 2010 at 2:51 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
> On Dec 2, 2010, at 2:45 PM, Vijay S. Mahadevan wrote:
>
>> Hi all,
>>
>> There is probably some minor inconsistency in my understanding but is
>> there a fundamental difference between the following two options ?
>>
>> // Option 1
>> ierr = MatMult(A, solution, temporaryvec) ;CHKERRQ(ierr);
>> ierr = VecAXPY(rhs, 1.0, temporaryvec) ;CHKERRQ(ierr);
>> // Option 2
>> ierr = MatMultAdd(A, solution, rhs, rhs) ;CHKERRQ(ierr);
>>
>> Here A is a shell matrix (serial) that has a routine defined to
>> perform MATOP_MULT only.
>>
>> I ask because Option 1 gives me the right result while Option 2
>> segfaults at the MatMultAdd line. My only logical conclusion is that I
>> need to define a MATOP_MULT_ADD or something similar for the shell for
>> this to work. Is this understanding correct ?
>
>   Yes, if your code will use a MatMultAdd() then you need to provide that to the shell matrix.
>
>> I implicitly assumed
>> that petsc recognizes that MATOP_MULT has been defined already and
>> since MatMultAdd only requires the action of a matrix on a vector to
>> perform its operation, should this not be computed by petsc
>> automatically ?
>
>   Sorry it doesn't though of course it could.
>
>> I really do not want to creat an extra vector here
>> with Option 1 since this occurs at a finer level in my calculation.
>> But is there any way that you would suggest I do this without extra
>> allocations ? Any comments or pointers will be much appreciated.
>
>   I suggest making a shell MatMultAdd_Mine() that does everything then
> also code a MatMult_Mine() that zeros the output vector and then calls directly MatMultAdd_Mine(). This way you'll
> have one code to routine but can handle both operations.
>
>   Barry
>
>
>
>>
>> Vijay
>
>