[petsc-users] Mat/Vec with empty ranks

[Florian Lindner]

Am 02.10.2017 um 21:04 schrieb Matthew Knepley:
> On Mon, Oct 2, 2017 at 6:21 AM, Florian Lindner <mailinglists at xgm.de <mailto:mailinglists at xgm.de>> wrote:
> 
>     Hello,
> 
>     I have a matrix and vector that live on 4 ranks, but only rank 2 and 3 have values:
> 
>     Doing a simple LSQR solve does not converge. However, when the values are distributed equally, it converges within 3
>     iterations.
> 
>     What can I do about that?
> 
>     I have attached a simple program and creates the matrix and vector or loads them from a file.
> 
> 
> There are a few problems with this program. I am attaching a cleaned up version. However, convergence still differs starting
> at iteration 2. It appears that LSQR has a problem with this system, or we have a bug that I cannot see.

Thanks for having a look at it!

And good to hear it's not by design. If I can be of any more help tracking this down, pleae let me know.

In the meantime, what could be a good way to work around this? This is admittedly a very malformed example. Is there a
way to force solving on a single CPU and then distribute the results resp. KSP object to the original parallel layout?
Of course, we would first try to solve in parallel, but we have little influence about the actual parallel layout, since
we are just a library and other solvers give us the data.

Best,
Florian