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https://issues.apache.org/jira/browse/LUCENE-7101?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15193359#comment-15193359
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Dawid Weiss commented on LUCENE-7101:
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Sorry for being dim, what's the scenario of hitting N^2 complexity here?
> OfflineSorter's merging is O(N^2) cost for large sorts
> ------------------------------------------------------
>
> Key: LUCENE-7101
> URL: https://issues.apache.org/jira/browse/LUCENE-7101
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Priority: Blocker
> Fix For: master, 6.0
>
>
> Our {{OfflineSorter}} acts just like Lucene, writing small initial
> segments of sorted values (from what it was able to sort at once in
> heap), periodically merging them when there are too many, and doing a
> {{forceMerge(1)}} in the end.
> But the merge logic is too simplistic today, resulting in O(N^2)
> cost. Smallish sorts usually won't hit it, because the default 128
> merge factor is so high, but e.g. the new 2B points tests do hit the
> N^2 behavior. I suspect the high merge factor hurts performance (OS
> struggles to use what free RAM it has to read-ahead on 128 files), and
> also risks file descriptor exhaustion.
> I think we should implement a simple log merge policy for it, and drop
> its default merge factor to 10.
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