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Subject: Re: [OMPI users] can't understand the mean of "ofud" in openmpi
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-12-02 10:34:39

On Dec 2, 2010, at 3:59 AM, 阚圣哲 wrote:

> When I use openmpi mpirun --mca btl <arg1>, I find arg1 can be ofud, self, sm, openib, but desn't explain those args.

"BTL" stands for "byte transfer layer" -- is the lowest networking software layer for the "ob1" MPI transport in Open MPI (ob1 is usually the default transport in Open MPI).

Each BTL supports a different kind of network:

- ofud: experimental UD-based OpenFabrics transport. I would not use this; it was developed as part of research and was never really finished.

- self: send-to-self (i.e., loopback to the same MPI process)

- sm: shared memory

- openib: generalized OpenFabrics transport.

Open MPI will automatically pick which BTL to use on a per-communication basis, based on which MPI process peer you are communicating with.

The "--mca btl ..." argument to mpirun restricts which BTLs Open MPI will use at run-time.

> I can't understand the mean of "ofud", what different between "ofud" and "openib",
> I also can't understand the different between "ibcm" and "rdmacm", when I use mpirun --mca btl_openib_cpc_include <arg2>.

There are 4 different ways for openib BTL to make connections across Open Fabrics networks:

- oob: the default ("out of band", meaning that it uses TCP sockets)
- xoob: the default when using Mellanox XRC ("out of band with XRC support")
- rdmacm: the default when using iWARP (because iWARP doesn't support OOB or XOOB)
- ibcm: not currently used; it's an IB-specific method that was never really finished

Usually, the right CM is just automatically picked -- you shouldn't need to manually select anything.

> maybe can publish a openmpi's document to explain those args and principle.

We are lacking in the documentation department; contributions would be greatly appreciated...

The README file has a bunch about BTLs; that may be helpful reading.

Jeff Squyres
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