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From "Shai Erera (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-5705) ConcurrentMergeScheduler/maxMergeCount default is too low
Date Sun, 25 May 2014 18:28:01 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-5705?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14008396#comment-14008396
] 

Shai Erera commented on LUCENE-5705:
------------------------------------

It depends what would you like to achieve. If you import documents that amount to 100 segments
and care only about the end result, i.e. a merged index (per the MP settings), then I am not
sure it will matter much if you first import w/o merging, and then call maybeMerge(). But
if you care about how fast DIH finishes importing, and are willing to let merges run in the
background while e.g. the index is searched, then disabling merges while you import data will
improve latency in that respect.

When I experimented with building indexes on Hadoop, I always disabled merges while the index
was built, and executed a special job afterwards. This prevented a lot of copying around HDFS.
Not saying this is your case, but sometimes it's useful to turn off merges, when you're executing
batch-oriented jobs.

> 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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