[petsc-users] 1D parallel split of dense matrix

[Barry Smith]

  If you are accessing things in the "matrix of vectors" you want to use MatDenseGetArray() and shift the pointer to any location in the matrix you wish to access. Using MatGetRow() or MatGetColumn() both are much slower than accessing the values directly in a dense array.

  Barry


> On Sep 17, 2020, at 4:36 PM, Yann Jobic <yann.jobic at univ-amu.fr> wrote:
> 
> Hi Stefano,
> You're right, i just switched the rows and colomns, and it's working fine. However, i've done this in order to have better performance when i access the rows, with MatGetRow. With this order, i need MatGetColumnVector, which is believe  is kind of slow, as stated in the comments of the implementation (but i couldn't find it again, maybe i was looking at an old implementation?).
> I still don't understand why i can not use the transpose of this matrix, as i give the right parallel decomposition. It should be a bijective operation no ?
> I think i'll be using the KSPMatSolve of Pierre, so i don't have to redevelop this part.
> Thanks a lot,
> Yann
> 
> 
> Le 9/17/2020 à 6:29 PM, Stefano Zampini a écrit :
>> Yann
>>  you want to have the number of columns equal to the number of rhs. Not the number of rows.
>> This is also consistent in terms of logical layouts. Simply create and dump the matrix this way and you can read in parallel splitting yhe rows ( ie the logical size per process of the vectors)
>> Il Gio 17 Set 2020, 18:09 Pierre Jolivet <pierre.jolivet at enseeiht.fr <mailto:pierre.jolivet at enseeiht.fr>> ha scritto:
>>    Hello Yann,
>>    This is probably not fully answering your question, but the proper
>>    way to solve a system with N RHS is _not_ to use KSPSolve(), but
>>    instead KSPMatSolve(), cf.
>>    https://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/KSP/KSPMatSolve.html.
>>    If you are tracking master (from the GitLab repository), it’s
>>    available out of the box. If you are using the release tarballs, it
>>    will be available in 3.14.0 scheduled to be released in a couple of
>>    days.
>>    If you want to know more about the current status of block solvers
>>    in PETSc, please feel free to have a look at this preprint:
>>    http://jolivet.perso.enseeiht.fr/article.pdf
>>    If you are using a specific PC which is not “multi-RHS ready”, see
>>    the list at the top of page 5, please let me know and I’ll tell you
>>    how easy to support it.
>>    Thanks,
>>    Pierre