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https://issues.apache.org/jira/browse/LUCENE-2357?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13287499#comment-13287499
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Adrien Grand commented on LUCENE-2357:
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Hi Otis,
Before this change, each doc map used ~ {{maxDoc * 32}} bits while they now use
~ {{maxDoc * lg(min(numDocs, numDeletedDocs))}} (where lg is the ceil of log in
base 2) bits. So even in the worst case (numDocs = numDeleted = maxDoc / 2),
the improvement is {{((31 - lg(maxDoc))/32}}. On a segment with
maxDoc=10000000, this is a 22% improvement. But the improvement is much better
when the number of deleted documents is close to 0 or to maxDoc. For example,
if your segment has maxDoc=10000000 and numDeletedDocs=100000, the improvement
({{32 - lg(min(numDocs, numDeletedDocs))/32}}) is close to 50%. If
numDeletedDocs=100, the improvement is close to 80%.
> 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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