You can run with -info and grep on GAMG, and send this.

If you are shifting a matrix then it can/will get indefinite. If it is just a mass matrix then Jacobi should converge quickly - does it?

On Fri, Nov 6, 2015 at 9:35 AM, Denis Davydov <davydden@gmail.com> wrote:

> On 6 Nov 2015, at 17:39, Barry Smith <bsmith@mcs.anl.gov> wrote:
>
>
> If it is a true mass matrix in the finite element sense of the word then it should be very well conditioned and one definitely would not use something like GAMG on. Jacobi + CG or maybe SSOR + CG should converge rapidly

That I understand and absolutely agree.
It just does not explain why GAMG would fail, especially on 4 cores and not on 8.

Regards,
Denis.