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From "Michael McCandless (JIRA)" <>
Subject [jira] [Updated] (LUCENE-5310) Merge Threads unnecessarily block on SerialMergeScheduler
Date Sun, 27 Apr 2014 13:17:15 GMT


Michael McCandless updated LUCENE-5310:

    Attachment: LUCENE-5310.patch

Arrrgh, you're right Simon.  I added an TestSerialMergeScheduler to
show this...

New patch, with another iteration, and a number of cleanups to CMS.
E.g. its merge method is no longer sync'd, its MergeThread class now
only runs one merge.  I use a semaphore to restrict running merges.

I also think CMS can deadlock (stall all merges) if you ever try to
decrease the maxMergeCount when too many merges were already running,
because the stall logic would get to a point where no thread would
ever be allowed to unstall.  So I changed it (and MergeScheduler) to
require the maxMergeCount up front.

An app that wants to do its own throttling can just set a high
maxMergeCount to CMS, and then throttle itself when
IW.getRunningMergeCount() is too high...

> Merge Threads unnecessarily block on SerialMergeScheduler
> ---------------------------------------------------------
>                 Key: LUCENE-5310
>                 URL:
>             Project: Lucene - Core
>          Issue Type: Improvement
>          Components: core/index
>    Affects Versions: 4.5, 5.0
>            Reporter: Simon Willnauer
>            Priority: Minor
>             Fix For: 4.9, 5.0
>         Attachments: LUCENE-5310.patch, LUCENE-5310.patch, LUCENE-5310.patch, LUCENE-5310.patch,
LUCENE-5310.patch, LUCENE-5310.patch
> I have been working on a high level merge multiplexer that shares threads across different
IW instances and I came across the fact that SerialMergeScheduler actually blocks incoming
thread is a merge in going on. Yet this blocks threads unnecessarily since we pull the merges
in a loop anyway. We should use a tryLock operation instead of syncing the entire method?

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