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From: Tony Ladd (ladd_at_[hidden])
Date: 2006-10-23 16:00:37

A couple of comments regarding issues raised by this thread.

1) In my opinion Netpipe is not such a great network benchmarking tool for
HPC applications. It measures timings based on the completion of the send
call on the transmitter not the completion of the receive. Thus, if there is
a delay in copying the send buffer across the net, it will report a
misleading timing compared with the wall-clock time. This is particularly
problematic with multiple pairs of edge exchanges, which can oversubscribe
most GigE switches. Here the netpipe timings can be off by orders of
magnitude compared with the wall clock. The good thing about writing your
own code is that you know what it has done (of course no one else knows,
which can be a problem). But it seems many people are unaware of the timing
issue in Netpipe.

2) Its worth distinguishing between ethernet and TCP/IP. With MPIGAMMA, the
Intel Pro 1000 NIC has a latency of 12 microsecs including the switch and a
duplex bandwidth of 220 MBytes/sec. With the Extreme Networks X450a-48t
switch we can sustain 220MBytes/sec over 48 ports at once. This is not IB
performance but it seems sufficient to scale a number of applications to the
100 cpu level, and perhaps beyond.


Tony Ladd
Professor, Chemical Engineering
University of Florida
PO Box 116005
Gainesville, FL 32611-6005

Tel: 352-392-6509
FAX: 352-392-9513
Email: tladd_at_[hidden]