[petsc-users] Dropping single entries in matrix insertion for block matrices

[Jed Brown]

"Smith, Barry F." <bsmith at mcs.anl.gov> writes:

>> On Dec 10, 2018, at 7:58 PM, Jed Brown <jed at jedbrown.org> wrote:
>> 
>> "Smith, Barry F. via petsc-users" <petsc-users at mcs.anl.gov> writes:
>> 
>>>   Well the thing is the indices you pass in are currently always
>>>   "blocked base", they work automatically for both blocked and non
>>>   blocked versions of MatSetValuesLocal(). 
>> 
>> What do you mean?
>
>    The indices passed to ISLocalToGlobalMappingCreate() are with respect to the block size of the mapping which has to match the block size of the matrix.

You could just create a scalar mapping.

>>  You pass block indices to MatSetValuesBlocked and
>> scalar indices to MatSetValues.  In MatSetValues_SeqBAIJ, for example
>> 
>>  for (k=0; k<m; k++) { /* loop over added rows */
>>    row  = im[k];
>>    brow = row/bs;
>> 
>>>   To support what you want you need to be able to pass in nonblock
>>>   based version of the indices and (as you noted) keep track of
>>>   whether you pass in the blocked or non blocked indices so you can
>>>   apply the scaling or not.
>>> 
>>>   Ok, give it a try, it does change the interface a bit but I suppose
>>>   is worth it since it introduces more functionality.
>> 
>> How would the interface change?
>
>     One would now be able to pass a non-blocked ISLocalToGlobalMapping to a blocked matrix. This is currently not allowed/supported. 

Is that an interface changing or just not erroring in a circumstance
where it previously did?  I normally think of interface change meaning
that previously correct code either behaves differently or fails.