Author: Keith Evans
Date: 17:58:18 07/15/03
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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
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