Author: Vincent Diepeveen
Date: 19:34:18 07/15/03
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On July 15, 2003 at 21:13:23, Jeremiah Penery wrote: >On July 15, 2003 at 20:19:34, Vincent Diepeveen wrote: > >>On July 15, 2003 at 15:24:19, Gerd Isenberg wrote: >> >>Gerd use it with a bigger hashtable. Not such a small >>table. >> >>400MB is really the minimum to measure. > >Why? > >Measuring 90MB, something like 99.65% of the accesses should be to RAM and not >cache. With 100MB, it's 99.8%. Yet when I measure those two things, I get a >whole 6.1ns latency difference according to your test. Even measuring only >20MB, 98.4% of the allocated memory can not be in cache. (All of this assumes >that the program takes up 100% of the cache, which it won't.) > >There's something wrong that causes memory access time to be reported much >higher when testing larger 'hashtable' sizes. Anything large enough to >overwhelm the cache should report similar, if not almost identical, results. >However, your program gives wildly different numbers. > >Trying to allocate 12500000 entries. In total 100000000 bytes > Average measured read read time at 1 processes = 183.935982 ns > >Trying to allocate 11250000 entries. In total 90000000 bytes > Average measured read read time at 1 processes = 177.806427 ns > >Trying to allocate 43750000 entries. In total 350000000 bytes > Average measured read read time at 1 processes = 253.592331 ns the only thing from which i was not sure was the RNG used for doing this type of tests, because the remainder gets used to index. >In the last test, I can't be completely sure I wasn't paging at all. I didn't >see the disk light flashing, but it's possible that this hit the disk more than >once, which would make the number look much higher than it should. You can see that by turning on perfmon. Also internetting at the same time has a bad influence here. Some online software is eating really some bandwidth. 90MB is sick little to use of course for such tests. I tested basically with sizes like 50GB at SGI. >Still, relative to the other results people have given, this is not so bad, >since I have only PC2100 memory (133MHz DDR). My experience is that when compared to others who did similar tests the numbers match when using 400MB or more as a cache size. If you want to find an explanation i guess the RNG is the weak chain in the test. That's why i had asked Dieters help there as in the past he said he knew a lot about RNGs. He didn't report back on that yet. We will see. The RNG is rotating bits a bit. usually that works well. I do not know though for the remainder whether it works so well.
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