[openib-general] mvapich-gen2 IA64 compile problem

[Roland Dreier]

    Peter> If that is the purpose it fails badly since I have both
    Peter> EM64T machines with PCI-X and AMD64 machines with
    Peter> PCI-express. Or am I missing something here?

Nope, the fact that it's messed up is what I was trying to point out.
>From a quick look at the code, it looks like you should set the
environment variable VIADEV_DEFAULT_MTU to "MTU1024" on PCI-X systems
and "MTU2048" on PCI Express systems.

In general, one wants to use the largest possible MTU to get maximum
performance.  However, Mellanox PCI-X HCAs have a quirk that causes an
MTU of 1024 to be faster than an MTU of 2048 for RC transport.  This
quirk does not exist in PCI Express HCAs, and presumably PathScale and
IBM HCAs also perform best with the maximum possible MTU.  So Mellanox
PCI-X HCAs are the only case where we want to use a lower MTU.

 - R.