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Subject: Re: Source code to measure it - results

Author: Keith Evans

Date: 09:58:33 07/16/03

Go up one level in this thread


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.



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