Author: Robert Hyatt
Date: 21:31:28 07/15/03
Go up one level in this thread
On July 16, 2003 at 00:02:35, Keith Evans wrote: >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? You don't need to. lm-bench runs at 128 byte stride by default, and that is beyond any L1/L2 linesize on any pentium-type machine made so far. It also tests to measure the cache line size to avoid producing data that would be badly skewed.
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