[petsc-dev] Extract sub-matrices efficiently

[Jed Brown]

Could you give some more information about your preconditioner?
Sequential extraction of many small submatrices is not high performance,
but perhaps we can restructure.

victor.magri at dicea.unipd.it writes:

>  
>
> Dear PETSc developers, 
> I have a matrix of the type MATMPIAIJ from which I want to extract small
> sub-matrices and sub-vectors (number of rows from 1 to 50 maximum)
> several times. This is a very fundamental step of a preconditioner that
> I'm writing. After that, I want to solve these sub-systems locally. 
>
> In my current implementation I use MatGetSubMatrices with
> MAT_INITIAL_MATRIX to extract the sub-matrices and MatGetRow plus some
> other artifacts to extract the sub-vectors. However, I see that this
> procedure is too costly, once it creates and destroys the output
> variables each time they are called. I thought about using the MAT_REUSE
> flag, but I can't since the nonzero pattern of the local sub-matrix may
> change from call to call. 
>
> Can you give me some hint on how to do this task efficiently? I
> appreciate your help! 
>
> -- 
>
> Victor A. P. Magri - PhD student
> Dept. of Civil, Environmental and Architectural Eng.
> University of Padova
> Via Marzolo, 9 - 35131 Padova, Italy 
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