[petsc-users] Does PETSc take advantage of symmetry in iterative solvers

Jed Brown jedbrown at mcs.anl.gov
Mon Nov 12 06:40:03 CST 2012

On Mon, Nov 12, 2012 at 3:35 AM, Alexander Grayver
<agrayver at gfz-potsdam.de>wrote:

> I remember similar question has been asked long time ago, but I can not
> find it.
> If I set MatSetOption(A,MAT_SYMMETRIC,**PETSC_TRUE), does PETSc take this
> into account when doing MatMult or similar operations?

The option does not change the format, but it does change some defaults
(e.g., it makes ASM symmetric instead of using RASM). If you want to use a
symmetric _format_, use MATSBAIJ.

> As far as I can see it does not (I'm not sure I look at the right place
> though). Could it really save time in principle?

Symmetric formats require more communication, but win on memory bandwidth.
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