[Beowulf] Lowered latency with multi-rail IB?
Nifty Tom Mitchell
niftyompi at niftyegg.com
Fri Mar 27 01:20:18 EDT 2009
On Thu, Mar 26, 2009 at 09:03:30PM -0700, Greg Lindahl wrote:
> On Thu, Mar 26, 2009 at 11:32:23PM -0400, Dow Hurst DPHURST wrote:
> > We've got a couple of weeks max to finalize spec'ing a new cluster. Has
> > anyone knowledge of lowering latency for NAMD by implementing a
> > multi-rail IB solution using MVAPICH or Intel's MPI?
> Multi-rail is likely to increase latency.
> BTW, Intel MPI usually has higher latency than other MPI
> If you look around for benchmarks you'll find that QLogic InfiniPath
> does quite well on NAMD and friends, compared to that other brand of
> InfiniBand adaptor. For example, at
> the lowest line == best performance is InfiniPath. Those results
> aren't the most recent, but I'd bet that the current generation of
> adaptors has the same situation.
What this implies is that NAMD is not purely
bandwidth limited. Rather it is limited by
other quickness issues. For the most
part multi-rail is a bandwidth enhancement play...
With multi-rail do double check the system bus (PCI-e)
bandwidth. If multi-rail is used determine how the
data is mux-ed between rails and what the impact of that
decision code path has on quickness and/or bandwidth.
If multi-rail is to go very fast MPI needs to
manage each rail/LID in productive ways for the
application. I doubt that this "productive way" has
a simple general one size fits all answer.
NAMD is clearly a "got to benchmark it" application!
Both the data link hardware and the MPI library integration with that
hardware are important...
The last table on Greg's URI pointer -- NAMD version is also important!
It is possible that NAMD.next will move to be more bandwidth limited
than it is today and then the notion of best interconnect/ platform
> -- Greg
> (yeah, I used to work for QLogic.)
T o m M i t c h e l l
Found me a new hat, now what?
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