whenever I ask about IB bandwidth, people always point fingers
at weather codes, which apparently are fond of doing the transpose
in multi-dimension FFT's using all-to-all. while convenient, this
seems a bit silly, since transpose is O(N) communications, not O(N^2).
Mark,
interconnect does matter.
I did not claim the opposite - I said that for small, cost-sensitive
clusters, it would be unusual to need IB's advantages (high bandwidth
and latency comparable to other non-Gb interconnects.)
in particular, I'm curious about the conventional wisdom about weather codes
and bandwidth.
Here is a solid benchmark using WRF, 128 cores,
Woodcrest 3.00GHz. System spec can be found at
http://www.spec.org/mpi2007/results/res2007q4/mpi2007-20071013-00029.html
Scali MPI Connect 5.6.2 using IB (IB as specified in the link above):
Success 127.wrf2 base mref ratio 21.03, runtime 370.653066
I was curious about this: you only used one DDR port; was that because
of lack of switch ports, or because WRF uses bandwidth <= DDR?
Scali MPI Connect 5.6.2 using Gbe (Broadcom NetXtreme II BCM5708 1000Base-T
(B2), Broadcom NetXtreme II Gigabit Ethernet Driver bnx2 v1.3.29, 9k MTU,
Switch? Jeff, can you fill in here, the system should be familiar to you):
Success 127.wrf2 base mref ratio=4.82, runtime=1618.248048
That's a pretty decent advantage to IB, isn't?
sure, and these are very fat nodes for which a fat interconnect is
appropriate for almost any workload that's not embarassing. but really
I wasn't suggesting that plain old Gb (bandwidth in particular) was
adequate for all possible clusters. I was questioning whether IB was
a panacea for small, cost-sensitive ones...
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