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

[Jed Brown]

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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