Author: Robert Hyatt
Date: 09:08:58 07/16/03
Go up one level in this thread
On July 16, 2003 at 01:39:34, Andrew Dados wrote: >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 >> >>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. >> >>Still, relative to the other results people have given, this is not so bad, >>since I have only PC2100 memory (133MHz DDR). > >Get rid of RNG.. do lookups every x+32 bytes (or so), after exhausting hashtable >inc(x) >-Andrew- That will fail on a PIV. It's L2 cache line is 128 bytes. You will see 1/4 the real latency, roughly, because 3 of every four probes will be a cache hit.
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