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Subject: Re: [OMPI users] Implementation of TCP v/s OpenIB (Eager and Rendezvous) protocols
From: Jeff Squyres (jsquyres) (jsquyres_at_[hidden])
Date: 2014-02-01 08:51:19

On Jan 31, 2014, at 2:49 AM, Siddhartha Jana <siddharthajana24_at_[hidden]> wrote:

> Sorry for the typo:
> ** I was hoping to understand the impact of OpenMPI's implementation of these protocols using traditional TCP.
> This is the paper I was referring to:
> Woodall, et al., "High Performance RDMA Protocols in HPC".
> On 31 January 2014 00:43, Siddhartha Jana <siddharthajana24_at_[hidden]> wrote:
> Good evening
> Is there any documentation describing the difference in MPI-level implementation of the eager and rendezvous protocols in OpenIB BTL versus TCP BTL ?

Unfortunately, there is not, sorry. Just the code. :-\

> I am only aware of the following paper. While this presents an excellent overview of how RDMA capabilities of modern interconnects can be leveraged for implementing these protocols, I was hoping to understand how OpenMPI implications of handling these protocols using traditional TCP.

The easiest way to think about it is that the TCP BTL could well be implemented with just the "send" method (and no "get" or "put" methods).

That being said, the TCP BTL does emulate the "put" method (meaning: there's obviously no hardware support for a direct data placement using a general socket in TCP like there is with OpenFabrics-style RDMA) simply because it allows us to be slightly more efficient on the receiver (IIRC; it's been a loooong time since I've looked at that code).

Jeff Squyres
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