Feature: Checkpointing for T8codeMesh

[jmark]

This PR adds checkpointing for T8codeMesh. By this, routines like save_mesh and load_mesh are supported.

Closes #2044

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 2.96610% with 229 lines in your changes missing coverage. Please review.

Project coverage is 87.12%. Comparing base (91eac3c) to head (573133a).

Files	Patch %	Lines
src/meshes/t8code_mesh.jl	0.00%	162 Missing ⚠️
src/meshes/mesh_io.jl	0.00%	58 Missing ⚠️
src/auxiliary/t8code.jl	58.33%	5 Missing ⚠️
src/callbacks_step/save_restart_dg.jl	0.00%	3 Missing ⚠️
src/callbacks_step/amr.jl	0.00%	1 Missing ⚠️

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HEAD has 4 uploads less than BASE

Flag	BASE (91eac3c)	HEAD (573133a)
unittests	25	21

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1980      +/-   ##
==========================================
- Coverage   96.23%   87.12%   -9.10%     
==========================================
  Files         462      462              
  Lines       37075    37233     +158     
==========================================
- Hits        35676    32439    -3237     
- Misses       1399     4794    +3395     

Flag	Coverage Δ
unittests	87.12% <2.97%> (-9.10%)	⬇️

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[benegee]

Great feature. Thank a lot!

[jmark]

Do you think the MPI failures are really unrelated?

No, I am not sure. I just know that we had stalling CI jobs before. And, looking trough the recent failures, I think it was not always the same elixir.

I get the feeling that the MPI tests are too big now and take too long. We probably have to split them up similar to the serial tests.

[JoshuaLampert]

I get the feeling that the MPI tests are too big now and take too long. We probably have to split them up similar to the serial tests.

Yes, could be related to OOM issues, cf. #1471.

[jmark]

I get the feeling that the MPI tests are too big now and take too long. We probably have to split them up similar to the serial tests.

Yes, could be related to OOM issues, cf. #1471.

I could narrow it down. It has something to do with Julia 10.1.4. With Julia 10.1.2 it does not stall. Investigating ...

[JoshuaLampert]

Are you able to reproduce the problem locally?

[jmark]

Yes! With Julia 1.10.2 the t8code MPI tests run successfully. However, with Julia 1.10.4 the MPI test for elixir_advection_restart.jl 2D stalls for whatever reason. Running the elixir with MPI directly (not wrapped in a test set) does not stall. No idea, right now, what's going on ...

[sloede]

Yes! With Julia 1.10.2 the t8code MPI tests run successfully. However, with Julia 1.10.4 the MPI test for elixir_advection_restart.jl 2D stalls for whatever reason. Running the elixir with MPI directly (not wrapped in a test set) does not stall. No idea, right now, what's going on ...

Are you sure it's related to the patch version bump? Are you using an identical Manifest.toml for both tests?

[jmark]

Yes! With Julia 1.10.2 the t8code MPI tests run successfully. However, with Julia 1.10.4 the MPI test for elixir_advection_restart.jl 2D stalls for whatever reason. Running the elixir with MPI directly (not wrapped in a test set) does not stall. No idea, right now, what's going on ...

Are you sure it's related to the patch version bump? Are you using an identical Manifest.toml for both tests?

Yes! Working from the exact same project folder. Just pointing the Julia binary to either 1.10.2 or 1.10.4.

[JoshuaLampert]

So it consistently stalls with Julia 1.10.4, but consistently works with Julia 1.10.2 in multiple runs? Did you monitor RAM usage during the simulation?

[jmark]

So it consistently stalls with Julia 1.10.4, but consistently works with Julia 1.10.2 in multiple runs? Did you monitor RAM usage during the simulation?

Yes! RAM usage is not out of ordinary.

[jmark]

I think I found the bug causing the stalls in the MPI runs. It was a silent memory leak/segfault. I added the fixes in the last commit. Furthermore, I changed the t8code C interface a tiny bit to simplify the code on Trixi side. This PR has to wait for the next breaking t8code release and specifically for the merge of this PR: DLR-AMR/t8code#1115.

I'll try to push for a major t8code release by the end of next week.