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Subject: Re: Source code to measure it - results

Author: Andrew Dados

Date: 22:39:34 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
>
>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-



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