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Subject: Re: [OMPI users] Unable to connect to a server using MX MTL with TCP
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-06-09 15:33:40

On Jun 5, 2010, at 7:52 AM, Scott Atchley wrote:

> I do not think this is a supported scenario. George or Jeff can correct me, but when you use the MX MTL you are using the pml cm and not the pml ob1. The BTLs are part of ob1. When using the MX MTL, it cannot use the TCP BTL.
> You only solution would be to use the MX BTL.

Sorry for the delayed reply.

Scott is correct; the MX MTL uses the "cm" PML. The "cm" PML can only use *one* MTL at a time (little known fact of Open MPI lore: "cm" stands for several things, one of which is "Connor MacLeod" -- there can only be one).

Here's a chunk of text from the README:

- There are three MPI network models available: "ob1", "csum", and
  "cm". "ob1" and "csum" use BTL ("Byte Transfer Layer") components
  for each supported network. "cm" uses MTL ("Matching Tranport
  Layer") components for each supported network.

  - "ob1" supports a variety of networks that can be used in
    combination with each other (per OS constraints; e.g., there are
    reports that the GM and OpenFabrics kernel drivers do not operate
    well together):
    - OpenFabrics: InfiniBand and iWARP
    - Loopback (send-to-self)
    - Myrinet: GM and MX (including Open-MX)
    - Portals
    - Quadrics Elan
    - Shared memory
    - TCP
    - SCTP
    - uDAPL

  - "csum" is exactly the same as "ob1", except that it performs
    additional data integrity checks to ensure that the received data
    is intact (vs. trusting the underlying network to deliver the data
    correctly). csum supports all the same networks as ob1, but there
    is a performance penalty for the additional integrity checks.

  - "cm" supports a smaller number of networks (and they cannot be
    used together), but may provide better better overall MPI
    - Myrinet MX (including Open-MX, but not GM)
    - InfiniPath PSM
    - Portals

  Open MPI will, by default, choose to use "cm" when the InfiniPath
  PSM MTL can be used. Otherwise, "ob1" will be used and the
  corresponding BTLs will be selected. "csum" will never be selected
  by default. Users can force the use of ob1 or cm if desired by
  setting the "pml" MCA parameter at run-time:

    shell$ mpirun --mca pml ob1 ...
    shell$ mpirun --mca pml csum ...
    shell$ mpirun --mca pml cm ...

Jeff Squyres
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