Simon Byrne

Do you want something like `reduce` from mpi4py: https://mpi4py.readthedocs.io/en/stable/reference/mpi4py.MPI.Comm.html#mpi4py.MPI.Comm.reduce

Yeah, their docs are a bit unclear, but the implementation is based on serializing `send` and `recv`: https://github.com/mpi4py/mpi4py/blob/18f16da45eaaacfba4cc0293964e1a6548d4c6a6/demo/reductions/reductions.py#L12-L41

oh, actually that is a demo, not what is actually used. It appears to be [this `reduce`](https://github.com/mpi4py/mpi4py/blob/0dccf615a66e73f2b334b6959e532a59015e1eed/src/mpi4py/MPI/Comm.pyx#L1609-L1617) which calls [`PyMPI_reduce`](https://github.com/mpi4py/mpi4py/blob/98174ae85f7ffaddf252b1e5343095208e29afd0/src/mpi4py/MPI/msgpickle.pxi#L1291-L1297), which in turn calls either - [`PyMPI_reduce_naive`](https://github.com/mpi4py/mpi4py/blob/98174ae85f7ffaddf252b1e5343095208e29afd0/src/mpi4py/MPI/msgpickle.pxi#L991-L994), which calls [`PyMPI_gather`](https://github.com/mpi4py/mpi4py/blob/98174ae85f7ffaddf252b1e5343095208e29afd0/src/mpi4py/MPI/msgpickle.pxi#L710-L758)...

> This is on MPI.jl 1.4.0 with OpenMPI 3.1.4 + UCX + pmi2 I assume you mean MPI.jl 0.14.0? I wonder if a newer UCX would work?

> As a follow-up questions, was it intentional to remove all typed pointers for This was fixed by #598

> * `MPI_Aint` to be of `ptrdiff_t` type, which is implementation defined (Q: isn't that a potential issue that it can vary?) > > * `MPI_Fint` to be of `int`...

Would you mind making a PR on the existing const files?

> What do you mean exactly? One that fixes the types I've pointed out above? yes

> Recently, version 0.19 was released. Since the major version number is 0, Julia's package manager flags this as incompatible with 0.18, as if this was a major release. I...

Oh the other thing is whether `Put` and `Get` should have `!` suffixes?