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Re: A short meditation about hash search performance

by Abigail-II (Bishop)
on Nov 16, 2003 at 02:54 UTC ( #307408=note: print w/replies, xml ) Need Help??

in reply to Re: Re: Re: A short meditation about hash search performance
in thread A short meditation about hash search performance

Abigail-II did a lot of benchmarking on it. Maybe Abigail-II would like to elaborate?
The benchmark was fairly simply: take about million different words, insert them in a hash, measure how long it takes and what the average chain length is. They are all common words, it's the combination of various English wordlists I once grabbed from a puzzle site, and long list of Dutch words. No specially prepared input. The average chain length was 1.27 on 5.8.0, 5.8.1, 5.8.2-RC1 and 5.8.2-RC2. The only interesting thing was the time - it took about 17.5 seconds on 5.8.0, 5.8.1 and 5.8.2-RC2, and less almost 4 seconds less on 5.8.2-RC1.
A same hash key list length of 1 for all hash keys, would be optimal if there were no other "costs" involved. However, the re-hashing of existing keys is not something to be done lightly, especially if the number of existing keys is high. So you need to find the best possible combination of same hash key list length and re-hashing. In that respect, the ideal same hash key list length is not 1!
I do not agree with the latter conclusion. The best possible combination of max chain length and re-hashing depends on the ratio number of inserts vs number of queries (for the sake of simplicity, let's not consider deletes). The lower this ratio is (that is, the more queries you have), the more time you can spend on inserts to get a better overal performance. That is, if you have enough queries, it pays to have max chain length of 1.


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