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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 Fri, 01 Jun 2012 16:33:23 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2357?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13287521#comment-13287521
] 

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

While working on LUCENE-4062, I had in mind that {{FAST}} (50%) would be ok for transient
data structures while {{COMPACT}} (0%) and {{DEFAULT}} (20%) would be better for big and long-living
structures depending on the performance requirements. However, it is true that the DocMap
might not be the bottleneck for merging (especially since this operation involves disk accesses).
I can try to run some benchmarks next week to find out whether {{COMPACT}} (or maybe {{DEFAULT}})
could be a better tradeoff.
                
> 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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