[openfabrics-ewg] [openib-general] IB MTU tunable for uDAPL and/or Intel MPI?

Michael Krause krause at cup.hp.com
Mon Jun 12 14:18:27 PDT 2006

At 10:44 AM 6/9/2006, Scott Weitzenkamp (sweitzen) wrote:
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>While we're talking about MTUs, is the IB MTU tunable in uDAPL and/or 
>Intel MPI via env var or config file?
>Looks like Intel MPI 2.0.1 uses 2K for IB MTU like MVAPICH does in OFED 
>1.0 rc4 and rc6, I'd like to try 1K with Intel MPI.

IB MTU should be set on a per path basis by the SM.  An application should 
examine the PMTU for a given path and take appropriate action - really only 
applies to UD as connected mode should automatically SAR 
requests.  Communicating PMTU to an application should not occur unless it 
is datagram based.   The same is true for iWARP where TCP / IP takes care 
of the PMTU on behalf of the ULP / application.  If you want to control 
PMTU, then do so via the SM directly which was the intention of the 
architecture and specification.


>From: openib-general-bounces at openib.org 
>[mailto:openib-general-bounces at openib.org] On Behalf Of Scott Weitzenkamp 
>Sent: Thursday, June 08, 2006 4:38 PM
>To: Tziporet Koren; openfabrics-ewg at openib.org
>Cc: openib-general
>Subject: RE: [openib-general] OFED-1.0-rc6 is available
>The MTU change undos the changes for bug 81, so I have reopened bug 81 
>With rc6, PCI-X osu_bw and osu_bibw performance is bad, and PCI-E osu_bibw 
>performance is bad.  I've enclosed some performance data, look at rc4 vs 
>rc5 vs rc6 for Cougar/Cheetah/LionMini.
>Are there other benchmarks driving the changes in rc6 (and rc4)?
>Scott Weitzenkamp
>SQA and Release Manager
>Server Virtualization Business Unit
>Cisco Systems
>·        Added mpi_alltoall fine tuning parameters
>·        Added default configuration/documentation file 
>·        Added shell configuration files  $MPIHOME/etc/mvapich.csh , 
>·        Default MTU was changed back to 2K for InfiniHost III Ex and 
>InfiniHost III Lx HCAs. For InfiniHost card recommended value is:
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