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To: Joe NYC who wrote (69502)	1/30/2002 11:44:52 AM
From: fyodor_	Respond to of 275872

Joe: If you look at the Hammer presentation, page 25, it appears that Sledgehammer will have a built-in 4 way switch (XBar or CrossBar). If this is in fact a swich (which I think it is), not some sort of hub or a daisychaining device, the performance penalty / latency would be less than daisychaining, since the number of hops is reduced. Clearly, the higher "wayness" of the switch / crossbar / whatever, the lower the average latency (i.e. the better the scalability). Daisy-chaining requires a 2-way switch and the average worst case latency is N/2 hops (with N being the total number of processors). 4-way allows a simple grid structure with a much lower average worst case latency, scaling with the square root of the number of processors. As an example, using 2-way switching, the average worst case latency for an 8-processor system would be 4 hops, whereas with 4-way switching, this could be reduced to a mere 2 hops. -fyo