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From "Dawid Weiss (JIRA)" <j...@apache.org>
Subject [jira] Updated: (LUCENE-2967) Use linear probing with an additional good bit avalanching function in FST's NodeHash.
Date Tue, 15 Mar 2011 13:37:29 GMT

     [ https://issues.apache.org/jira/browse/LUCENE-2967?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Dawid Weiss updated LUCENE-2967:
--------------------------------

    Attachment: LUCENE-2967.patch

Linear probing in NodeHash.

> Use linear probing with an additional good bit avalanching function in FST's NodeHash.
> --------------------------------------------------------------------------------------
>
>                 Key: LUCENE-2967
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2967
>             Project: Lucene - Java
>          Issue Type: Improvement
>            Reporter: Dawid Weiss
>            Assignee: Dawid Weiss
>            Priority: Trivial
>             Fix For: 4.0
>
>         Attachments: LUCENE-2967.patch
>
>
> I recently had an interesting discussion with Sebastiano Vigna (fastutil), who suggested
that linear probing, given a hash mixing function with good avalanche properties, is a way
better method of constructing lookups in associative arrays compared to quadratic probing.
Indeed, with linear probing you can implement removals from a hash map without removed slot
markers and linear probing has nice properties with respect to modern CPUs (caches). I've
reimplemented HPPC's hash maps to use linear probing and we observed a nice speedup (the same
applies for fastutils of course).
> This patch changes NodeHash's implementation to use linear probing. The code is a bit
simpler (I think :). I also moved the load factor to a constant -- 0.5 seems like a generous
load factor, especially if we allow large FSTs to be built. I don't see any significant speedup
in constructing large automata, but there is no slowdown either (I checked on one machine
only for now, but will verify on other machines too).

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