Documentation: random Open MPI v4.1.x --> v5.0.x changes

[jsquyres]

Originally taken from the "to-do" section in the Open MPI docs:

• Document what MPI-4.0 items are supported, e.g., sessions, persistent communication, ...
• What specifically does --disable-io-romio do?
• Do we still have AMCA files?
• Do we still have --tune files?
• Document v5.0.x’s backwards-compatibility relationship with v4.x (including ABI and mpirun command line options). It will likely be:
  • C bindings are ABI compatible
  • Fortran mpif.h should be ok (but probably bears checking).
  • Fortran module ABIs may have changed. These definitely need checking (e.g., w.r.t. MPI_Status and friends…?).
• MPI-1 deleted functions are always included in the library.
  • This is a reversal from what we previously publicly stated would happen in Open MPI v5.0.x
  • Per Brian: the behavior is the same as Open MPI 4.x. The deleted functions are not declared in mpi.h unless Open MPI was compiled with --enable-mpi1-compatibility.
• Add a note somewhere about “These HTML docs are available at docs.open-mpi.org and in distribution tarballs in docs/_build/html/index.html.
• Make “setting an MCA param” docs prominent in the doc (this already exists somewhere, but we need to make it prominent).

William (Removed FAQ item as it's covered under #11363)

[wckzhang]

For - Make “setting an MCA param” docs prominent in the doc (this already exists somewhere, but we need to make it prominent). how do you want it to be more prominent? It's not too hard to find on sidebar it's under "8. The Modular Component Architecture (MCA)" as "8.2 Setting MCA parameter values." It seems to be fairly prominent unless your intention is to have it be one of the numbered items on the sidebar

[jsquyres]

@wckzhang Put yourself in the mindset of an Open MPI newbie: will they find these docs? Setting an MCA param is a common thing to do. Should we put it in one of the "quick start" pages (or have a new quick start for it, or ...?). Suggestions and proposals are welcome here.

[wckzhang]

Hmm, I'm not sure a user would know what an MCA parameter is unless they read about MCA, maybe in the quick start guide there can be a section after launch MPI such as "Customize run-time options." which can link to the setting MCA parameter doc

[wckzhang]

#11459 - For Make “setting an MCA param” docs prominent in the doc (this already exists somewhere, but we need to make it prominent).