[petsc-users] Unique number in each element of a DMPlex mesh

[Matthew Knepley]

On Mon, Jan 22, 2024 at 1:57 PM Berend van Wachem <berend.vanwachem at ovgu.de>
wrote:

> Dear Matt,
>
> Thanks for your quick response.
> I have a DMPlex with a polyhedral mesh, and have defined a number of
> vectors with data at the cell center. I have generated data
> for a number of timesteps, and I write the data for each point to a file
> together with the (x,y,z) co-ordinate of the cell center.
>
> When I want to do a restart from the DMPlex, I recreate the DMplex with
> the polyhedral mesh, redistribute it, and for each cell
> center find the corresponding (x,y,z) co-ordinate and insert the data that
> corresponds to it. This is quite expensive, as it
> means I need to compare doubles very often.
>
> But reading your response, this may not be a bad way of doing it?
>

It always seems to be a game of "what do you want to assume?". I tend to
assume that I wrote the DM and Vec in the same order, so when I load them
they match. This is how Firedrake I/O works, so that you can load up on a
different number of processes (https://arxiv.org/abs/2401.05868).

So, are you writing a Vec, and then redistributing and writing another Vec?
In the scheme above, you would have to write both DMs. Are you trying to
avoid this?

  Thanks,

     Matt


> Thanks,
>
> Berend.
>
> On 1/22/24 18:58, Matthew Knepley wrote:
> > On Mon, Jan 22, 2024 at 10:49 AM Berend van Wachem <
> berend.vanwachem at ovgu.de <mailto:berend.vanwachem at ovgu.de>> wrote:
> >
> >     Dear Petsc-Team,
> >
> >     Is there a good way to define a unique integer number in each element
> >     (e.g. a cell) of a DMPlex mesh, which is in the same location,
> >     regardless of the number of processors or the distribution of the
> mesh
> >     over the processors?
> >
> >     So, for instance, if I have a DMPlex box mesh, the top-right-front
> >     corner element (e.g. cell) will always have the same unique number,
> >     regardless of the number of processors the mesh is distributed over?
> >
> >     I want to be able to link the results I have achieved with a mesh
> from
> >     DMPlex on a certain number of cores to the same mesh from a DMPlex
> on a
> >     different number of cores.
> >
> >     Of course, I could make a tree based on the distance of each element
> to
> >     a certain point (based on the X,Y,Z co-ordinates of the element),
> and go
> >     through this tree in the same way and define an integer based on
> this,
> >     but that seems rather cumbersome.
> >
> >
> > I think this is harder than it sounds. The distance will not work
> because it can be very degenerate.
> > You could lexicographically sort the coordinates, but this is hard in
> parallel. It is fine if you are willing
> > to gather everything on one process. You could put down a p4est, use the
> Morton order to number them since this is stable for a
> > given refinement. And then within each box lexicographically sort the
> centroids. This is definitely cumbersome, but I cannot
> > think of anything else. This also might have parallel problems since you
> need to know how much overlap you need to fill each box.
> >
> >    Thanks,
> >
> >        Matt
> >
> >     Thanks and best regards, Berend.
> >
> > --
> > What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to
> > which their experiments lead.
> > -- Norbert Wiener
> >
> > https://www.cse.buffalo.edu/~knepley/ <
> http://www.cse.buffalo.edu/~knepley/>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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