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From "Shawn Heisey (JIRA)" <>
Subject [jira] [Commented] (LUCENE-5705) ConcurrentMergeScheduler/maxMergeCount default is too low
Date Sun, 25 May 2014 14:43:01 GMT


Shawn Heisey commented on LUCENE-5705:

bq. Wait, this comment should also be in trunk?

It very likely is in trunk.  I was just trying to be precise about where I actually looked,
in case trunk says something slightly different and others happen to be looking too.

bq. Hmm that should not have been the case; if you turn on IW infoStream, CMS tells you when
it's pausing a large merge

I've never actually done this.  I will turn on infostream and start a new full rebuild of
the entire 96 million document index.  Those infostreams will be available after several hours,
and they ought to be very large.

I just know that when importing millions of records from a database, if you don't increase
maxMergeCount, the incoming thread will stall long enough for JDBC to kill the connection.
 If smaller merges were really running first, then it seems like we would never be over the
threshold long enough for the connection to die -- my smallest merges would probably complete
in less than a second, and the next size up would only take a few seconds.  When I first noticed
the problem, I clocked one merge-caused indexing pause at over eight minutes.

> ConcurrentMergeScheduler/maxMergeCount default is too low
> ---------------------------------------------------------
>                 Key: LUCENE-5705
>                 URL:
>             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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