Feature request: improved zero-copy non-contiguous send

[pascal-boeschoten-hapteon]

Hello,

With XPMEM, MPICH supports zero-copy intra-node sends for non-contiguous datatypes.
But AFAICT there are some restrictions:

• Can't use with hindexed or struct datatypes.
• The size of the datatype is limited by a header field (defined by MPIDI_POSIX_MAX_AM_HDR_SIZE), limiting the complexity of the data that can be sent at once.
• MPICH does not release XPMEM segments. If the user doesn't reuse buffers, MPICH will accumulate segments until it hits an XPMEM limit and crashes.

Could anything be done about these limitations? And could it be made to work inter-node with e.g. UCX active message?

Kind regards,
Pascal Boeschoten