[petsc-dev] attaching field information to PetscLayout?

[Barry Smith]

On May 14, 2011, at 2:47 PM, Dmitry Karpeev wrote:

> On Sat, May 14, 2011 at 11:51 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>> 
>>    I have a proposal for a moderate/major change for how we handle knowledge of subfields/splits in PETSc.
>> 
>>    Currently one can call PCFieldSplitSetIS() or PCFieldSplitSetFields() to indicate the splitting of vectors(fields) into subvectors(subfields).  But, in fact, the knowledge of what splits make sense really comes in the generation of the vectors and matrices and it is bad that the user needs to stash that information somewhere and then bring it out and attach it to the PCFieldSplit. It is especially annoying if one is using a fieldsplit inside, say a multigrid preconditioner, and one has to somehow get that information down into the inner PC.
>> 
>>   I propose an alternative. All Vecs and Mats currently have associated PetscLayouts and when a Vec or Mat is duplicated the new Vec or Mat has that same PetscLayout. I propose each PetscLayout have associated with it an optional set of IS indicating the subfields/subvectors (note that this is sort of already true when you set a blocksize larger than 1 the PCFieldSplit will by default use the strides for subfields). Thus PCFieldSplit can get directly from its matrix the appropriate default splits.   Similarly the VecStrideXXX() operations can be made more general becoming VecSubVecXXX(). We also add VecGetSubVec() to get appropriated sized subvectors easily.
>> 
>>   The end result will be a relatively small change in the PETSc API and for most users little or absolutely no change, but the power to compose easily will really increase.
>> 
>>   Comments, objections, improvements?
> 
> Can these split ISs have communicators different from that of
> PetscLayout (this is something I use in GASM,
> for example)?

   Beats me.  The ISs that define the splits could have a subcommunicator I guess that is not either 1 or all.


   Barry

> 
>> 
>>    Do this before or after the next release? (I guess the conventional wisdom answer would be after?).
>> 
>> 
>>    Barry
>> 
>> 
>> BTW: It is true that parallel vectors and matrices share the PetscLayout from their parent object (via VecDuplicate/MatDuplicate) but this is not true to sequential Vec and Mat. Before adding Field information to PetscLayouts I would need to change all the sequential duplicates to get a reference to their parents PetscLayout instead of creating a new one.
>> 
>>