Author: Robert Hyatt
Date: 11:36:56 07/16/03
Go up one level in this thread
On July 16, 2003 at 12:58:33, Keith Evans wrote: >On July 16, 2003 at 10:29:07, Robert Hyatt wrote: > >>On July 16, 2003 at 00:44:34, Keith Evans wrote: >> >>>On July 16, 2003 at 00:29:43, Robert Hyatt wrote: >>> >>>>On July 16, 2003 at 00:05:29, Keith Evans wrote: >>>> >>>>>On July 15, 2003 at 23:35:30, Robert Hyatt wrote: >>>>> >>>>>>On July 15, 2003 at 23:05:37, Vincent Diepeveen wrote: >>>>>> >>>>>>> >>>>>>> >>>>>>>Now i can disproof again the 130ns figure that Bob keeps giving here for dual >>>>>>>machines and something even faster than that for single cpu (up to 60ns or >>>>>>>something). Then i'm sure he'll be modifying soon his statement something like >>>>>>>to "that it is not interesting to know the time of a hashtable lookup, because >>>>>>>that is not interesting to know; instead the only scientific intersting thing is >>>>>>>to know is how much bandwidth a machine can actually achieve". >>>>>>> >>>>>> >>>>>> >>>>>>What is _interesting_ is the fact that you are incapable of even recalling >>>>>>the numbers I posted. >>>>>> >>>>>>to wit: >>>>>> >>>>>>dual xeon 2.8ghz, 400mhz FSB. 149ns latency >>>>>> >>>>>>PIII/750 laptop, SDRAM. 125ns. >>>>>> >>>>>>Aaron posted the 60+ ns numbers for his overclocked athlon. I assume his >>>>>>numbers are as accurate as mine since he _did_ run lm_bench, rather than >>>>>>something with potential bugs. >>>>>> >>>>>>I can post bandwidth numbers if you want, but that has nothing to do with >>>>>>latency, as those of us understanding architecture already know. >>>>>> >>>>> >>>>>Can you run lmbench and give the latency numbers for different stride sizes? >>>>>Then you could quote numbers from cache,... >>>>> >>>> >>>>Here's my laptop data. L1 seems to be 4 clocks. L2 9 clocks, memory >>>>at 130ns. This is a PIII/750mhs machine with SDRAM. I just ran it again >>>>to produce these numbers. >>>> >>>> >>>> >>>>Host OS Mhz L1 $ L2 $ Main mem Guesses >>>>--------- ------------- --- ---- ---- -------- ------- >>>>scrappy Linux 2.4.20 744 4.0370 9.4300 130.2 >>>> >>>>>In the lmbench paper they have a nice graph like this. >>>> >>>> >>>>Is the above what you want? >>> >>>I think that it's as close as you're going to get. The most important thing is >>>that 130 [ns] is the largest number. And wouldn't that be a little bit >>>pessimistic even for chess hash tables? >> >> >>I don't think so, although, in the case of crafty, the actual latency is >>about 1/3 of that, since I read three positions and you would ammortize the >>latency over those three positions rather than just over one. > >That's what I meant - the 130 [ns] number is pessimistic given that you really >have an average latency 1/3 of that. Correct. There are also other issues. IE instructions are read sequentially for the most part so their latency is way lower. Other things that have lengths > 4 bytes also get a lower apparent latency. I think 130 (laptop) or 150 (dual xeon) are pretty good upper bounds on worst-case latency, where real-world numbers are going to be lower, since there are not a lot of random-access reads to deal with anyway. One hash probe per node is not a lot of data to worry about.
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