[petsc-dev] Very Useful Description of MAT Types in Petsc Index

[Jacob Faibussowitsch]

Hello,

In keeping with PETSc design it would be nice to have *more* detail for all MAT implementations explaining in what the letters stand for, and use-cases it might be useful for. For example:

> MATAIJSELL <https://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Mat/MATAIJSELL.html#MATAIJSELL> = "AIJSELL" - A matrix type to be used for sparse matrices. This matrix type is identical to MATSEQAIJSELL when constructed with a single process communicator, and MATMPIAIJSELL otherwise. As a result, for single process communicators, MatSeqAIJSetPreallocation <https://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Mat/MatSeqAIJSetPreallocation.html#MatSeqAIJSetPreallocation>() is supported, and similarly MatMPIAIJSetPreallocation <https://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Mat/MatMPIAIJSetPreallocation.html#MatMPIAIJSetPreallocation>() is supported for communicators controlling multiple processes. It is recommended that you call both of the above preallocation routines for simplicity.

I am no complete beginner but I am no computational matrix expert either. I have no idea what “SELL” is. Obviously googling "mat sell format” gives less than useful results. Other such types are MATAIJCRL, MATAIJMKL, MATAIJPERM, MATADJ. 

Smallest change could be as simple as mirroring MATSBAIJ, which is clear and concise:

> MATMPISBAIJ <https://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Mat/MATMPISBAIJ.html#MATMPISBAIJ> = "mpisbaij" - A matrix type to be used for distributed symmetric sparse block matrices, based on block compressed sparse row format. Only the upper triangular portion of the "diagonal" portion of the matrix is stored. For complex numbers by default this matrix is symmetric, NOT Hermitian symmetric. To make it Hermitian symmetric you can call MatSetOption <https://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Mat/MatSetOption.html#MatSetOption>(Mat <https://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Mat/Mat.html#Mat>, MAT_HERMITIAN <https://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Mat/MatOption.html#MatOption>); 

Clearest case would be:

> MATMPISBAIJ = "mpisbaij" - MATrix MPI Symmetric Block AIJ. A matrix type to be used for distributed symmetric sparse block matrices, based on block compressed sparse row format.
> Only the upper triangular portion of the "diagonal" portion of the matrix is stored. For complex numbers by default this matrix is symmetric, NOT Hermitian symmetric. To make it Hermitian symmetric you can call MatSetOption(Mat, MAT_HERMITIAN); 


Each format is useful in its own case, but people can’t use things if they don’t know what it is!

Best regards,

Jacob Faibussowitsch
(Jacob Fai - booss - oh - vitch)
Cell: (312) 694-3391

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