Manipulating matrix directly

[torhans]

Hi

I there a place to read about how I can manipulate the left hand matrix (and right hand vector) in a way that is consistent with with parallel execution? And effective.

For now I just need to couple 2 linear systems (poisson equation), but I would like to understand the process so that I can do it also for more complicated problems.

Thanks
Torben

[ordinary-slim]

By manipulate do you mean do a custom assembly? This test shows how to do a custom assembly in python.

You want to couple 2 Poisson problems on the same mesh or different meshes?

[torhans]

Thanks, I’ll take a closer look at it later, I think it will be informative when I dig into it, but after a brief look it reminds me of one of the things that confuses me.

Those scatter forward and backwards what do they do, when do I call them? My internet searches so far has not been very fruitful, so a pointer to some explanation would be helpful.

Regarding the 2 poissons; what I hope to do is to split a mesh along an internal subdomain boundary and make a discontinuity in the potential.

Thanks

[francesco-ballarin]

@torhans

Regarding the 2 poissons; what I hope to do is to split a mesh along an internal subdomain boundary and make a discontinuity in the potential.

There are plenty of ways to do that without fiddling with the low level matrices, for instance:

• https://multiphenics.github.io/
• Discontinuity at interface using mixed domains

[dokken]

@torhans

Those scatter forward and backwards what do they do, when do I call them? My internet searches so far has not been very fruitful, so a pointer to some explanation would be helpful.

They are covered in
https://scientificcomputing.github.io/mpi-tutorial/notebooks/dolfinx_MPI_tutorial.html