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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-5705) ConcurrentMergeScheduler/maxMergeCount default is too low
Date Sun, 25 May 2014 14:08:01 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-5705?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14008344#comment-14008344
] 

Michael McCandless commented on LUCENE-5705:
--------------------------------------------

bq. I did notice the following comment in the 4x branch

Wait, this comment should also be in trunk?

bq. Older versions seemed to prefer running the largest merge to completion before doing the
smaller ones. 

Hmm that should not have been the case; if you turn on IW infoStream, CMS tells you when it's
pausing a large merge (confusingly, via the abort method) to let smaller merges finish.

> ConcurrentMergeScheduler/maxMergeCount default is too low
> ---------------------------------------------------------
>
>                 Key: LUCENE-5705
>                 URL: https://issues.apache.org/jira/browse/LUCENE-5705
>             Project: Lucene - Core
>          Issue Type: Bug
>          Components: core/other
>    Affects Versions: 4.8
>            Reporter: Shawn Heisey
>            Assignee: Shawn Heisey
>            Priority: Minor
>             Fix For: 4.9
>
>         Attachments: LUCENE-5705.patch, LUCENE-5705.patch, dih-example.patch
>
>
> The default value for maxMergeCount in ConcurrentMergeScheduler is 2.  This causes problems
for Solr's dataimport handler when very large imports are done from a JDBC source.
> What happens is that when three merge tiers are scheduled at the same time, the add/update
thread will stop for several minutes while the largest merge finishes.  In the meantime, the
dataimporter JDBC connection to the database will time out, and when the add/update thread
resumes, the import will fail because the ResultSet throws an exception.  Setting maxMergeCount
to 6 eliminates this issue for virtually any size import -- although it is theoretically possible
to have that many simultaneous merge tiers, I've never seen it.
> As long as maxThreads is properly set (the default value of 1 is appropriate for most
installations), I cannot think of a really good reason that the default for maxMergeCount
should be so low.  If someone does need to strictly control the number of threads that get
created, they can reduce the number.  Perhaps someone with more experience knows of a really
good reason to make this default low?
> I'm not sure what the new default number should be, but I'd like to avoid bikeshedding.
 I don't think it should be Integer.MAX_VALUE.



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