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Subject: Re: Source code to measure it

Author: Robert Hyatt

Date: 17:06:36 07/15/03

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On July 15, 2003 at 17:14:45, Gerd Isenberg wrote:

>On July 15, 2003 at 09:33:39, Robert Hyatt wrote:
>
>>On July 15, 2003 at 06:24:58, Vincent Diepeveen wrote:
>>
>>>On July 14, 2003 at 16:07:27, Robert Hyatt wrote:
>>>
>>>You measure the latency with those benches of sequential reads.
>>
>>No.  lm-bench does _random_ reads and computes the _random-access_
>>latency.
>>
>>Don't know why you have a problem grasping that.
>>
>>
>>>So already opened cache lines you can get data faster from than
>>>random reads to memory.
>>
>>That also makes no sense.  Perhaps you mean "already opened memory
>>rows"?
>>
>>
>>>
>>>Random reads to memory are about 280 ns at single cpu P4 and about 400ns at dual
>>>P4s.
>>
>>No they aren't.
>>
>
>Bob, i found nothing wrong with Vincent's code. He does N-random hashreads and
>aggregates the time used. I thought about some factor 2 error - but found no one
>so far. Random Hashreads, like chess programs do.
>
>1e9 random hash reads take 265 seconds (including ~60 seconds overhead) on my
>athlon-pc, however latency is defined. Any explanation? Any systematical error
>or assumption? What does lm-bench do, to measure latency?
>
>Regards,
>Gerd


It is possible to cause _other_ problems.  IE you can push the instructions
in the loop out of cache, for one thing.  There are others.  The best numbers
I have seen come from lm-bench.  It was not a quick and dirty program, it has
a lot of research behind it to address specific issues that were pointed out
over a period of a year.

It is very easy to use a "low impedence probe" if you know what that means.  It
actually affects the circuit it is measuring.

200+ns seems way high to me, when the chip latency is less than 1/3 of that.

again, I'd run lm-bench on your box to see what it says, then you have to
reconcile the differences.




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