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Subject: Re: [OMPI devel] Amateur Guidance
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2008-11-03 14:40:17

On Nov 3, 2008, at 10:39 AM, Eugene Loh wrote:

> Main answer: no great docs to look at. I think I've asked some OMPI
> experts and that was basically the answer they gave me.

This is unfortunately the current state of the art -- no one has had
time to write up good docs.

Galen pointed to the new papers -- our main PML these days is
"ob1" (teg died a long time ago).

PML = Point to point messaging layer; it's basically the layer that is
right behind MPI_SEND and friends.

The ob1 PML uses BTL modules underneath. BTL = Byte transfer layer;
individual modules that send bytes back and forth over individual
transports (e.g., shared memory, TCP, openfabrics, etc.). There's a
BTL for each of the major transports that we support. The protocols
that ob1 uses are described nicely in the papers that Galen sent, but
the specific function interfaces are only best described in ompi/mca/

Alternatively, we have a "cm" PML which uses MTL modules underneath.
MTL = Matching transport layer; it's basically for transports that
expose very MPI-like interfaces (e.g., elan, tports, PSM, portals,
MX). This cm component is extremely thin; it basically provides a
shim between Open MPI and the underlying transport.

The big difference between cm and ob1 is that ob1 is a progress engine
that tracks multiple transport interfaces (e.g., shared memory, tcp,
openfabrics, ...etc. -- and therefore potentially multiple BTL module
instances) and cm is a thin shim that simply translates between OMPI
and the back-end interface -- cm will only use *ONE* MTL module
instance. Specifically: it is expected that the one MTL module will
do all the progression, striping, ...or whatever it wants to do to
move bytes from A to B by itself (very little/no help at all from
OMPI's infrastructure).

Does that help some?

Jeff Squyres
Cisco Systems