[petsc-users] Question about matrix permutation

[Jed Brown]

On Fri, 29 Jan 2010 13:48:57 -0600, Barry Smith <bsmith at mcs.anl.gov> wrote:
>     **If you have a matrix with say block size of 4 and store it with  
> BAIJ and do factorizations and solvers and it takes N seconds then  
> solve the same system with factorization and solvers with AIJ it will  
> take at least 1.5 * N seconds.

If the AIJ system uses full blocks, then it's much less clear since
inodes would be used.  I've still found this number to be about right,
but not due to the standard "memory bandwidth required for the matrix
entries and column indices" performance model.  I have a hard time
explaining it since the vector access is almost identical.  Perhaps it
is due to poor prefetch of matrix entries?

Jed