Run CI on aarch64 with Drone

[giordano]

With CPUs like A64FX, aarch64 is an interesting platform for HPC. I think
it'd be good to tests MPI.jl works here. I just ran the tests on Isambard 2
and they are successful.

However, I have a small application which I tried to run on a MacBook M1
(aarch64-apple-darwin) and it crashes at
https://github.com/JuliaLang/julia/blob/58ffe7e3ed3a93a9d816097548e785284f57fbd4/src/codegen.cpp#L5531-L5536
in a call to MPI.Reduce!. Tests are successful also here, so perhaps they
don't capture this error (or the error lies somewhere else), but I'll need to
produce a minimal working example to get a useful test out of it.

If you want to go on with this one, someone will need to enable this repository
at https://cloud.drone.io/.

---

Edit: regarding the error I got, I'm using a custom reduction, Valentin pointed out this is known to not work at the moment on non-Intel platforms and tests about them are skipped.

[simonbyrne]

What was the code doing? It sounds like you were using a custom reduction operator: these aren't supported on ARM or PPC (we currently disable tests on those platforms, see #353).

#404 had some ideas to workaround this.

[giordano]

Yes, as I wrote in the "Edit", I was using a custom reduction

[giordano]

Julia v1.6 on aarch64 is failing with

Test Failed at /drone/src/test/test_io_shared.jl:22
  Expression: MPI.File.get_position_shared(fh) == 0
   Evaluated: 9 == 0
ERROR: LoadError: There was an error during testing
in expression starting at /drone/src/test/test_io_shared.jl:22

Interestingly enough, Julia v1.3 is successful 😕

[simonbyrne]

Yeah, that happens intermittently. It's unclear what the consistency semantics are for shared file pointers.

[simonbyrne]

Thanks!

[giordano]

BTW, I forgot to mention that tests are successful on aarch64-apple-darwin with ENV["JULIA_MPI_BINARY"]="OpenMPI_jll" (Edit: I actually mentioned tests are successful already 😅 not that I used OpenMPI_jll)