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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2329) Use parallel arrays instead of PostingList objects
Date Tue, 23 Mar 2010 09:08:27 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2329?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12848626#action_12848626
] 

Michael McCandless commented on LUCENE-2329:
--------------------------------------------

Sweet, this looks great Michael!  Less RAM used and faster indexing (much less GC load) --
win/win.

It's a little surprising that the segment count dropped from 41 -> 32?  Ie, how much less
RAM do the parallel arrays take?  They save the object header per-unique-term, and 4 bytes
on 64bit JREs since the "pointer" is now an int and not a real pointer?  But, other things
use RAM (the docIDs in the postings themselves, norms, etc.) so it's surprising the savings
was so much that you get 22% fewer segments... are you sure there isn't a bug in the RAM usage
accounting?

> Use parallel arrays instead of PostingList objects
> --------------------------------------------------
>
>                 Key: LUCENE-2329
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2329
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Michael Busch
>            Assignee: Michael Busch
>            Priority: Minor
>             Fix For: 3.1
>
>         Attachments: lucene-2329.patch, lucene-2329.patch, lucene-2329.patch
>
>
> This is Mike's idea that was discussed in LUCENE-2293 and LUCENE-2324.
> In order to avoid having very many long-living PostingList objects in TermsHashPerField
we want to switch to parallel arrays.  The termsHash will simply be a int[] which maps each
term to dense termIDs.
> All data that the PostingList classes currently hold will then we placed in parallel
arrays, where the termID is the index into the arrays.  This will avoid the need for object
pooling, will remove the overhead of object initialization and garbage collection.  Especially
garbage collection should benefit significantly when the JVM runs out of memory, because in
such a situation the gc mark times can get very long if there is a big number of long-living
objects in memory.
> Another benefit could be to build more efficient TermVectors.  We could avoid the need
of having to store the term string per document in the TermVector.  Instead we could just
store the segment-wide termIDs.  This would reduce the size and also make it easier to implement
efficient algorithms that use TermVectors, because no term mapping across documents in a segment
would be necessary.  Though this improvement we can make with a separate jira issue.

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