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From "Shawn Heisey (JIRA)" <>
Subject [jira] [Commented] (LUCENE-5705) ConcurrentMergeScheduler/maxMergeCount default is too low
Date Sat, 24 May 2014 15:54:01 GMT


Shawn Heisey commented on LUCENE-5705:

bq. Do you know why merges can't keep up in your use case? E.g. are you throttling the merge

I have the TMP equivalent of mergeFactor 35, and I'm importing 16 million docs from MySQL
into each shard.  The final shard size is over 18GB.  I've seen the same thing happen to others
with the default mergeFactor.  ramBufferSizeMB is 48.  I have no throttling config.  The index
is on a RAID10 volume comprised of six 1TB SATA disks with a 1MB stripe size, so it's not
slow.  It just takes several minutes to merge at the gigabyte scale.

Recently I added autoCommit at 25000 docs with openSearcher=false, which I think does reduce
the size of each initial segment a little bit, but I have not tried again with the default
maxMergeCount.  I've had mine at 6 for years now, and others have had their import problems
fixed with that setting.

> 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
> 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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