[petsc-dev] Shifting in ICC, changes?

[Jed Brown]

On Fri, Dec 3, 2010 at 18:00, Hong Zhang <hzhang at mcs.anl.gov> wrote:

> We replaced old numerical factorization and MatSolve with faster data
> structure,
> then cleaned up MatPivotCheck() (still using previous algorithm).
> Changes, such as
> changeset:   15909:fbc1ba94d8d9
> user:        hong at hong-zhangs-macbook-air.local
> date:        Wed May 12 22:17:30 2010 -0500
> files:       src/mat/impls/aij/seq/aijfact.c
> description:
> replace macros with inline function MatPivotCheck() for inplace lu (aij
> format)
>
> could accelerate performance.
>

I know the data structures changed, but I see significantly different
results (number of iterations, presence of negative pivots) for ICC(0) with
SBAIJ(2).  The new results are better, but I had an explanation for poor
convergence that involved negative pivots needing to be shifted.  This was
the case at that time, but now I don't see negative pivots, and unless I'm
misunderstanding what I'm seeing, no shift is being applied now.  If that is
really the case, then I'm seeing more than a data structure change.  If a
shift is still being applied, I should be able to find where that happens.

I currently see only positive determinants at sbaijfact.c:1122 (current
petsc-dev).

    ierr = Kernel_A_gets_inverse_A_2(diag,shift);CHKERRQ(ierr);

If I hg update -d 2010-02-01 (changeset aef32a8bffe7), then on the
corresponding line (sbaijfact.c:964), I see some determinants on the order
of -1.e-9 (only when the problem starts to get hard).

I'm trying to understand what changed.

Jed
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20101203/aa5902a8/attachment.html>