[petsc-users] [SLEPc] non-deterministic behaviour in GHEP with Krylov-Schur

[Denis Davydov]

> On 8 Mar 2016, at 12:38, Jose E. Roman <jroman at dsic.upv.es> wrote:
> 
> As you can see, the eigenvector for the first eigenvalue (which is simple) is the same in both runs. The rest are multiple eigenvalues, so the corresponding eigenvectors are not uniquely determined simply by normalization. This means that, for instance, any linear combination of v1,v2,v3 is an eigenvector corresponding to 4.71385. Parallel

There is no questions about this.

> computation implies slightly different numerical error in different runs, and that is why you are getting different eigenvectors. But in terms of linear algebra, both runs are
> correct.

Frankly, i don’t see a reason for that. 
Assuming that the partition of degrees-of-freedom is the same and the number of MPI cores is the same,
the result should be deterministic on the same machine.

Unless some random number seed is changing and thus influence which eigenvectors are obtained for the degenerate eigenvalue...


Regards,
Denis