[petsc-users] sparse direct solver and number of nonzero elements

[Jed Brown]

No, a different ordering will be computed, and the ordering with the
sparser matrix could even lead to more fill.
On Dec 9, 2011 4:13 PM, "Xiangdong Liang" <xdliang at gmail.com> wrote:

> On Fri, Dec 9, 2011 at 7:02 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> > On Dec 9, 2011, at 5:55 PM, Xiangdong Liang wrote:
> >
> >> Hello everyone,
> >>
> >> I am solving Ax=b with sparse direct solver Pastix. I have two
> >> equivalent A's (upto these zero entries): A1 and A2. A1 is generated
> >> with ignor_zero_entries and A2 is without this option. For example A1
> >> has 9 millions nonzeros, while A2 has 10 millions zeros. When I solve
> >> them with Pastix, I found the time for solving sparser A1 actually is
> >> longer (10%--20% worse) than A2. Does anyone have thoughts on this?
> >
> >   Completely possible and not particularly surprising. The amount of
> work required for sparse LU depends in an incrediably complicated way on
> the nonzero structure of the matrix, it is only very very minorly related
> to the number of nonzeros in the matrix.
> >   One fun thing to check would be the number of nonzeros in the factor
> of A1 and the number of nonzeros in the factor of A2. (I'm not sure if
> Pastix has a way to check this).
>
> Should the nnz in the factor of A1 be the same as nnz in the factor of
> A2 since A1 and A2 are the same matrices except some zeros (due to
> ignore_zero_entries in the matrix assembling)?
>
> Xiangdong
>
> >
> >   Barry
> >
> >> Thanks.
> >>
> >> Xiangdong
> >>
> >> P.S. The time I count is only for Spare LU solving (not including the
> >> matrix assembling time).
> >
>
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