[petsc-dev] Matrix Options shown too many times in -help

[Jed Brown]

Currently Matrix Options are printed in MatView_Private which is called from
many places, e.g.

MatAssemblyEnd
MatZeroRowsColumns
MatZeroRows
MatLUFactorNumeric
MatCholeskyFactorNumeric

The problem here is that in time-stepping or a multigrid solve, it's common
to have these functions called hundreds or thousands of times which is
really overwhelming to -help output. One solution would be to store on a
per-matrix basis whether help has been displayed and only view it once (this
implies caching all the -mat_view_* options in the Mat, or calling with
PetscOptionsGetBool after the first time). The options could even be moved
to MatSetFromOptions, but that doesn't get called nearly as much as we would
probably like.

There is also the somewhat deeper problem of multiple objects with the same
prefix (often no prefix). This is most visible for matrices inside of
multigrid and/or fieldsplit. It is easy to have more than 20 matrices with
no prefix so even if the multiple-output problem above was solved, it would
still be fairly overwhelming. I think this is somewhat harder to fix because
it would imply a per-prefix log of whether the option has been printed. I
think that the one special case of no-prefix Mat would already be a
significant usability improvement.
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