[petsc-users] GAMG scaling

[Jed Brown]

Mark Adams via petsc-users <petsc-users at mcs.anl.gov> writes:

> Anyway, my data for this is in my SC 2004 paper (MakeNextMat_private in
> attached, NB, this is code that I wrote in grad school). It is memory
> efficient and simple, just four nested loops i,j,I,J: C(I,J) =
> P(i,I)*A(i,j)*P(j,J). In eyeballing the numbers and from new data that I am
> getting from my bone modeling colleagues, that use this old code on
> Stampede now, the times look reasonable compared to GAMG. This is optimized
> for elasticity, where I unroll loops (so it is really six nested loops).

Is the A above meant to include some ghosted rows?

> In thinking about this now, I think you want to make a local copy of P with
> rows (j) for every column in A that you have locally, then transpose this
> local thing for the P(j,J) term. A sparse AXPY on j. (My code uses a
> special tree data structure but a matrix is simpler.)

Why transpose for P(j,J)?