[petsc-users] MatMPIBAIJSetPreallocationCSR i and j indices

[Jed Brown]

Matteo Parsani <parsani.matteo at gmail.com> writes:
> 2 - why those routines *should not be used* when writing a fresh code or a
> large re-factoring?

Barry wrote a great rationale, but consider an example: almost all
third-party linear algebra libraries (and some special features in
PETSc) require scalar (non-blocked) matrix formats, but PETSc has good
implementations of other operations like matrix multiplication and
incomplete factorization using BAIJ format.  Thus with some solution
methods, you want a BAIJ matrix and with other methods, you need an AIJ
matrix.  If you try to build the backend data format directly, you'll
either need multiple versions to specifically support each format or
there will need to be a copy, which doubles the peak memory usage
required for the matrix.  If you just use MatXAIJSetPreallocation and
MatSetValuesBlocked, then you can choose the matrix format at run-time,
in which case all solvers become available without the extra memory
usage.

There are many more examples.  It is bad software design for a component
to depend on more than it must.  Depend on generic interfaces instead of
internal formats.  It will make your software more powerful and more
maintainable.
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