[Beowulf] using two separate networks for different data streams

Mark Hahn hahn at physics.mcmaster.ca
Fri Jan 27 21:36:03 EST 2006


> > The network for MPI should in many cases have low latency, so is expensive
> > (Myrinet, InfiniBand, etc.) in regards of Ethernet.

I tend to think of IB as mainly for high-bandwidth, since afaikt its 
latency isn't even as good as myri 2g (mx, 3 us).  to say nothing of 
myri 10g, quadrics, numalink, sci.

> > The I/O, NFS and
> > system network does not need low latency,

well, sort of.  I can imagine workloads (perhaps bio-database stuff)
that might take real advantage of lower-latency networks for IO.  but it's
also quite easy to see IO workloads that would exceed the bandwidth that 
a single GBE offers (say 80 MB/s).  and there are storage systems that
can actually drive many high-bandwidth links (Lustre, DDN, etc).
I'm jaded, but I do think of IB as more a faster, cheaper SAN than 
as a MPI-oriented low-latency interconnect ;)

> > and so for bargain cost can be
> > added, with the additional ground that it provides a control network to
> > tweak the nodes remotely when the expensive low latency network is down.

I guess.  which MPI-oriented nets are commonly down?  I haven't had any
problems with myri 2g or quadrics.

> Is there a way of characterizing in what proportion a given application
> relies on OpenMP, and how much the application depends on MPI (and hence
> MPI network latency) - other than speaking with application developers
> to get their intuitive feel, that is?  :)

we use logs from our job scheduler(s).  queues are separate for serial,
threaded and parallel/mpi.

> We're looking to buy a Gigabit Ethernet network for the MPI on this, but
> if that's obscenely high latency, and the primary application the
> cluster's being purchased for is heavily dependent on MPI, then we might

well, "heavily dependent" isn't really the same as "latency sensitive".
I find surprisingly many uses who are not unhappy with gigabit until they
scale above moderate (say, 16-64) numbers of CPUs.

regards, mark hahn.

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