Docs: is btl framework an MPI layer framework

[Fruchix]

Hi, I've been reading the OpenMPI 5 documentation about the internal frameworks, and the btl framework is referenced as a "Miscellaneous" framework.

I thought it was an MPI layer framework, as I have read in The ABCs of Open MPI - Part 2 from the Easy Build Tech Talks (slides 29-30).

Can someone confirm that or explain why it is referenced as a Miscellaneous framework?

Here is the documentation link and the corresponding file:

• https://docs.open-mpi.org/en/v5.0.x/developers/frameworks.html
• https://github.com/open-mpi/ompi/blob/main/docs/developers/frameworks.rst

Thanks for your help

[bosilca]

The Byte Transport Layer (BTL) is certainly not an MPI layer framework, it doesn't understand the MPI semantics, nor datatype. It simply provides a basic API (and implementation) to establish connections to peers and move bytes between two peers, using a specific hardware, in an efficient way. As such, it does not have communication protocols, but instead it provides the foundation (Active Messages and one sided semantics) more sophisticated framework, in OMPI case the P2P Management Layer (PML) and One-Sided Components (OSC), can be built upon.

[jsquyres]

@bosilca is 💯 right. That being said, way back in the day, the BTL used to be located in the MPI layer (i.e., the ompi/ folder tree). At some point, we moved the BTL down into the OPAL layer (i.e., the opal/ folder tree). I don't remember exactly when that happened vs. when we made the ABCs slides/presentation. It's possible that those slides were made when the BTL was still an "MPI" framework (i.e., in the ompi/ tree). Or it's possible that we made a mistake. 😄

[Fruchix]

Ok, thank you both for the detailed answer!

Indeed, those slides were made for v4.x so it might explain the difference.