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From "Shawn Heisey (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-5705) ConcurrentMergeScheduler/maxMergeCount default is too low
Date Sat, 24 May 2014 18:31:02 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-5705?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14008183#comment-14008183
] 

Shawn Heisey commented on LUCENE-5705:
--------------------------------------

bq. The purpose of maxMergeCount is to put back pressure on ongoing indexing when merges are
falling behind.

This is definitely something that I had not considered.  Solr DIH is probably the one place
where this back pressure can be considered a bad thing by default.  If we decide that the
best solution does involve changes to Solr DIH rather than the scheduler, I will move the
issue to the Solr project.

At the very least, I think it's a good idea to add maxMergeCount to the example-DIH indexConfig
section and include a comment.


> 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
>
>
> 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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