Cross-reference MPI_COMM_SPLIT_TYPE and MPI_WIN_ALLOCATE_SHARED in Chapters 7 and 12 (MPI-4 numbering)

[tonyskjellum]

Problem

A common idiom is to use MPI_COMM_SPLIT_TYPE then MPI_WIN_ALLOCATE_SHARED ; you then know where you can do
load/store programming on the new, sub-communicators that emerge.

Proposal

Add cross-references in both chapters, and an example in Chapter 12.

For MPI_COMM_TYPE_SHARED is the only pre-defined type; purely defined currently to work together.

Changes to the Text

At a high level:

1. A forward reference in Chapter 7 to MPI_WIN_ALLOCATE_SHARED in Chapter 12.
2. A backward reference in CHapter 12 to MPI_COMM_SPLIT_TYPE w/ type MPI_COMM_TYPE_SHARED
3. Example code in C/Fortran in Chapter 12 demonstrating the paradigm.

Impact on Implementations

None.

Impact on Users

Users will know more easily that these operations go hand in hand.

References and Pull Requests

TBD.

[tonyskjellum]

@wgropp this is based on feedback from users for wanting to be able to understand this use case.

[GuillaumeMercier]

@tonyskjellum: the forward reference to MPI_WIN_ALLOCATE_SHARED in MPI_COMM_TYPE_SHARED is already there.
I'm otherwise OK with a backward reference in chapter 12 to chapter 7.

[GuillaumeMercier]

Is there a particular reason as to why the Chapter 7 Chair was not assigned to this?

[GuillaumeMercier]

@tonyskjellum : also, the same behaviour can be achieved with MPI_COMM_TYPE_HW_GUIDED and the keyval mpi_shared_memory. In case of backward reference and example, I would like this to be mentioned/appear too.

[wgropp]

I'm ok with this change, but someone needs to prepare a PR.