I’m implementing an Allreduce algorithm inside the mca/coll framework.

The algorithm I’m implementing needs each node to send at each step of the computation only a part of the vector (like a ring allreduce), but the size of this part and the position can change.

More importantly, I should be able to send non contiguous part of the vector.

I was thinking about using a custom mpi datatype, the problem is that the normal MPI_Datatype … is just the final implementation on what is inside the mca (and the others) module(s).

I need to know how to create such custom datatype using only the low level calls of the module (no MPI_…).

For the send and recv part I was thinking about using the ompi_datatype_sendrcv (implemented in ompi/datatype/ompi_datatype_sendrcv.c) but suggestions even on this are welcome.