Author: Matt Taylor
Date: 11:41:56 12/05/02
Go up one level in this thread
On December 05, 2002 at 10:25:48, Robert Hyatt wrote: >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. I think most DDR-based systems have about 100ns latency. Opteron is quoted as 80ns latency -total-. Just another reason to look forward... http://www.amd.com/us-en/assets/content_type/DownloadableAssets/Optimization_-_Tim_Wilkens.pdf -Matt
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