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From "Adrien Grand (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-2357) Reduce transient RAM usage while merging by using packed ints array for docID re-mapping
Date Wed, 06 Jun 2012 22:01:23 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2357?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13290477#comment-13290477
] 

Adrien Grand commented on LUCENE-2357:
--------------------------------------

I ran a quick test that indexes a few millions of documents with only one field (index, not
stored, not analyzed, no terms vectors, ...) with different ratios of deleted documents, ram
buffer sizes (between 1 and 50 MB) and merge factors (between 3 and 20). The global speedup
with {{PackedInts.FAST}} was between 0.2% and 1.7% compared to {{PackedInts.COMPACT}} (although
I ran this test on a low-end computer, other people might have slightly better results with
the {{FAST}} version on a better machine). This is probably not worth the potential memory
overhead. Would someone disagree to replace {{FAST}} with {{COMPACT}} for the docmaps instantiation?
                
> Reduce transient RAM usage while merging by using packed ints array for docID re-mapping
> ----------------------------------------------------------------------------------------
>
>                 Key: LUCENE-2357
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2357
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: core/index
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>            Priority: Minor
>             Fix For: 4.0
>
>         Attachments: LUCENE-2357.patch, LUCENE-2357.patch, LUCENE-2357.patch, LUCENE-2357.patch,
LUCENE-2357.patch
>
>
> We allocate this int[] to remap docIDs due to compaction of deleted ones.
> This uses alot of RAM for large segment merges, and can fail to allocate due to fragmentation
on 32 bit JREs.
> Now that we have packed ints, a simple fix would be to use a packed int array... and
maybe instead of storing abs docID in the mapping, we could store the number of del docs seen
so far (so the remap would do a lookup then a subtract).  This may add some CPU cost to merging
but should bring down transient RAM usage quite a bit.

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