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From "Adrien Grand (JIRA)" <>
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 16:16:23 GMT


Adrien Grand commented on LUCENE-2357:

This is the theoretical improvement. However, in order not to slow merging down too much,
I instantiate the {{PackedInts.Mutable}} that holds the doc map with {{acceptableOverheadRatio=PackedInts.FAST=50%}}
(see LUCENE-4062), so the actual improvement might be a little worse than the theoretical
improvement. If you are more interested in memory usage than in merge speed, you could still
reach the theoretical improvement by replacing {{PackedInts.FAST}} with {{PackedInts.COMPACT}}
in {{}}.
> Reduce transient RAM usage while merging by using packed ints array for docID re-mapping
> ----------------------------------------------------------------------------------------
>                 Key: LUCENE-2357
>                 URL:
>             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,
> 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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