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Subject: Re: Source code to measure it - there is something wrong

Author: Keith Evans

Date: 21:02:35 07/15/03

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On July 15, 2003 at 22:25:01, Vincent Diepeveen wrote:

>On July 15, 2003 at 20:58:18, Keith Evans wrote:
>
>>On July 15, 2003 at 20:30:04, Vincent Diepeveen wrote:
>>
>>>On July 15, 2003 at 20:08:57, Robert Hyatt wrote:
>>>
>>>>On July 15, 2003 at 17:58:01, Gerd Isenberg wrote:
>>>>
>>>>>Ok, i think there is one problem with Vincent's cache benchmark.
>>>>>
>>>>>There are two similar functions DoNrng and DoNreads. DoNrng is used to mesure
>>>>>the time without hashread. But the instructions has the potential of faster
>>>>>execution due to less dependencies and stalls. It may execute parts of two loop
>>>>>bodies of DoNrng interlaced or simultaniesly - that is not possible in DoNreads.
>>>>>Therefore the time for N DoNrng is not the time used inside the N DoNrng loop,
>>>>>and maybe much faster.
>>>>
>>>>That is also certainly possible.  This kind of "problem" is highly
>>>>obfuscated, as you can see.  It requires a lot of analysis, by a lot of
>>>>people, to see the flaws.  That's why lm-bench is so respected.  It was
>>>>written, a paper was written about it, another paper was written that
>>>>pointed out some flaws, some of which were fixed and some of which were
>>>>not really flaws.  But it has been pretty well looked at by a _lot_ of
>>>>people.
>>>>
>>>>Other latency measures may well be as accurate, but until they "pass the
>>>>test of time and exposure" they are hard to trust.
>>>
>>>For sure my test shows that it isn't 130 ns. It's more like 280 ns for 133Mhz
>>>DDR ram. not sure whether you got RDRAM in your machine or 100Mhz DDR ram. but
>>>you for sure aren't at 130ns random memory latency there.
>>>
>>>If instructions get paired better or worse is not real interesting. It is nice
>>>when it measures in 0.1 ns accurate but if it is an error of 0.5 ns like it is
>>>now (assuming no other software is disturbing) then that is not a problem for me
>>>knowing the actual latencies lie in 210 for 150Mhz ram (just 300MB cache which
>>>is definitely too little) to 280 for 133Mhz ram (with 500MB cache) at P4 to
>>>nearly 400 ns for dual P4/K7s with DDR ram 133Mhz.
>>
>>Vincent,
>>
>>What do you think is wrong with the lmbench lat_mem_rd (memory read latency)
>>benchmark?
>>
>>Keith
>
>That's measuring the sequential latency. So if you first read in an
>array[60000000] the first 8 bytes then the bytes 8..15 then bytes 16..23 and so
>on. That is faster for memory.
>
>However in computerchess we do not lookup position 0 1 2 3 4 5 6 in memory, but
>we search. So we get semi random lookups which are unpredictable of course.
>
>So then you get confronted with extra latency for technical RAM reasons. It is
>therefore interesting for computerchess to measure the average random latency.
>Of course like Gerd says the real latency is even cooler but it won't be far off
>from the RASML test.
>
>Best regards,
>Vincent

Can't you increase the stride size in lmbench to get around this?



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