[petsc-users] inplace MatPtAP

[Jed Brown]

Patrick Lacasse <patrick.m.lacasse at gmail.com> writes:

> I have to compute a product Pt*A*P.
>
> My projection matrix P won't change the structure of A and I don't need to
> keep the original A.
> Is it possible to do it inplace (modify A instead of creating a new C)?

No.

> Do you think it is feasible to expend MatPtAP to mpiaij and/or sbaij
> matrices?

You want A to be SBAIJ and P MPIAIJ?  (Block formats for P usually don't
make sense except for MAIJ because P often has non-square blocks.)

> There is more implementation of MatMatMult.
> Is it possible to do Pt*A and A*P directly into A (without creating a copy)?

No.  (Not from laziness, but because the semantics of matrix-matrix
multiply don't work that way.  We could allocate the memory internally
and free the old copies, but that amount of seeming mutability is a
terrible interface for users and it doesn't reduce peak memory usage.)
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