Info on MPI Datatypes

[jdinan]

Problem

Today we cannot apply MPI Info hints to datatypes.

Proposal

Add the following functions:

int MPI_Type_commit_with_info(MPI_Datatype *datatype, MPI_Info info);
int MPI_Type_get_info(MPI_Datatype datatype, MPI_Info *info_used);

Changes to the Text

Changes to add the functions proposed above, specify that info does not propagate from one datatype object to another, and specify which info can be applied to datatypes.

Impact on Implementations

Implementations can ignore info and implement these functions with little effort.

New info keys and assertions can enable optimizations within the MPI library, e.g. to specify the memory kind for a buffer or request that the MPI library launch GPU data packing kernel for a noncontiguous buffer.

Impact on Users

Info would be opt-in. Users can provide more information to the MPI library to enable performance optimizations.

References and Pull Requests

• Hybrid WG Issue: Accelerator Info Keys

[schulzm]

Interesting idea - one question, though: what should be the rules when new datatypes are created from multiple datatypes with the info attached? Will they be merged?

Perhaps it could be easier to apply the info object during the commit (i.e., a MPI_Type_commit_with_info) ? This way the info is only valid for that committed type and the assembly of new types would be not affected.

[jdinan]

I think MPI_Type_commit_with_info would be a cleaner API. In general, I think we would want to maintain the design principle that info does not automatically propagate from one MPI object to another.