CI: Update unit tests from ubuntu-20.04 to ubuntu-24.04

[DilumAluthge]

GitHub retired their Ubuntu 20 image (for GitHub-hosted runners) on April 15, 2025. Therefore, we are forced to update.

https://github.blog/changelog/2025-01-15-github-actions-ubuntu-20-runner-image-brownout-dates-and-other-breaking-changes/

[giordano]

Ok, the problem is that in the jobs using mpich from Ubuntu repository with mpiexec -np 4 ... we're executing four processes which have MPI_LOCALNRANKS=4 but MPI.Comm_size(MPI.COMM_WORLD)=1 and all processes have rank 0. I'm quite lost about why this is happening.

[giordano]

Oh well, I get the same with a C program:

Hello world from processor fv-az1964-164, rank 0 out of 1 processors
Hello world from processor fv-az1964-164, rank 0 out of 1 processors
Hello world from processor fv-az1964-164, rank 0 out of 1 processors
Hello world from processor fv-az1964-164, rank 0 out of 1 processors

Instead with openmpi:

Hello world from processor fv-az1660-563, rank 3 out of 4 processors
Hello world from processor fv-az1660-563, rank 0 out of 4 processors
Hello world from processor fv-az1660-563, rank 2 out of 4 processors
Hello world from processor fv-az1660-563, rank 1 out of 4 processors

What's wrong with mpich?!?!

[giordano]

I think we're running into pmodels/mpich#7064. I don't think we can really do anything about this.