[petsc-users] MatGetValues / MatGetRow

[Smith, Barry F.]

> On May 28, 2018, at 12:15 AM, Marius Buerkle <mbuerkle at web.de> wrote:
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> No M is stored as MPIAIJ, as all but the A11 and A33 blocks are very sparse. I want to add  A11 and A33 which are calculuted seperatly using MATSOLVERELEMENTAL into the bigger matrix M and then use a sparse solver on M.

   Hmm, how big are the A11 and A33? Who calculates them separately?

   I would use MatConvert() on these two "separate" matrices as I mentioned before and call MatSetValues() with the resulting dense matrices to put the 
"dense" values into the sparse M matrix.


   Barry

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> > On May 27, 2018, at 11:23 PM, Marius Buerkle <mbuerkle at web.de> wrote:
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> > Thanks for the swift reply. I want to from, eventually invert, a sparse matrix of the form M=[A11 A12 A13 , A21 A22 A23 , A31 A32 A33], where A11 and A33 are dense square matrices which have a (much) smaller dimension than the other (sparse) sub matrices of M. A11 and A33 are obtained from a separate calculation.
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> I don't understand how this relates to your elemental question?
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> How are you storing the M matrix? As Elemental matrix?
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> Barry
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> >> Hi !
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> >> Are MatGetValues / MatGetRow implemented for MATELEMENTAL?
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> > A quick check seems to say no.
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> >> Or is there another way to access the elements of an ELEMENTAL matrix?
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> > There is no obvious way to access the elements of an Elemental matrix except to directly access the elemental data structure (which presumably is hairy) or to MatConvert(A,MATDENSE,MAT_INITIAL_MATRIX,&Adense); and then access in the dense format (note this conversion is expensive and needs to do communication).
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> > Why do you wish to access the values directly instead of use them indirectly with the API, such as matrix-vector products, factorizations etc?
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> > Barry
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> >> best,
> >> marius
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