Author: Robert Hyatt
Date: 07:25:48 12/05/02
Go up one level in this thread
On December 05, 2002 at 01:16:46, Jeremiah Penery wrote: >On December 04, 2002 at 23:15:03, Robert Hyatt wrote: > >>On December 04, 2002 at 20:29:52, Bob Durrett wrote: >> >>> >>>The recent threads shed some light on the issue of when one is more important >>>than another, but the answer is sketchy and seems to be "depends." >>> >>>For current chess-playing programs, which is more important? Latency or >>>bandwidth? Why? >>> >>>Is the answer different if multiple processors are used? >>> >>>Bob D. >> >>First, chess engines have a bad habit of doing real random-access probes to >>memory for things like hashing. That is latency-dependent rather than bandwidth >>because we are not reading large chunks, but small scattered 16-32 byte >>blocks... >> >>That said, programs do need some bandwidth as you have to keep the cpu fed with >>instructions and data and that stuff resides in memory unless the program is >>small enough to tuck away in L1/L2/L3 cache. >> >>multiple processors increases the bandwidth requirement. Two cpus require >>twice the bandwidth as one, and generally, high-end server boards provide two- >>way memory interleaving to double the bandwidth. >> >>Given the choice of 2x the bandwidth or 1/2 the latency, I'd go for reducing >>the latency. But that is basically impossible. Latency as been stuck at >>100-120ns for 20+ years now... > >I'm not sure why you persist in giving this 100-120ns number, when several >credible sources have said that the figure of today is no more than 75ns. Did you see the thread on rec.games.chess.computer? 75ns is often quoted but that is the latency in the SIMM/DIMM. That is not the _complete_ path from the CPU to RAM. 100 is *very* good. 120-135 is much more typical. If you can find a box that shows 75ms latency for random reads of one byte, I'd certainly like to see it. Don't do random reads of 128 bytes as that amortizes latency over several words of data and makes it look better... I want to know exactly how long I have to wait after I do something like mov eax, [ebx] before I can use that result in eax.
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