[petsc-users] creating a DM with fewer processors than available?

[Jed Brown]

Frederik Treue <frtr at fysik.dtu.dk> writes:

> Hi,
>
> I am solving 2D problems using petsc, but for some diagnostics I need 1D
> objects. This works fine, except if I use many processors for a
> relatively small grid: say gridsize 512x512, with 256 processors. In the
> 2D DM, this is not a problem, I get 32x32 blocks on each processor. But
> in the 1D object petsc allocates 2 points to each processor, and since I
> have a stencil with of 2, it croaks! Is there any way around this, ie.
> use less processors for a designated DM?

No, you can create the DMDA on a subcommunicator, but then it can't be
composed into the outer solver.  How is it currently being used?  Can it
be eliminated from the solve and just used as an auxiliary component to
solve the 2D problem?

Unfortunately, there is currently no way to do a reduced distribution on
the global communicator.  If the number of points is less than 1000, you
might just use DMRedundant.
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