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From "Michael McCandless (JIRA)" <>
Subject [jira] [Commented] (LUCENE-5705) ConcurrentMergeScheduler/maxMergeCount default is too low
Date Mon, 26 May 2014 06:38:01 GMT


Michael McCandless commented on LUCENE-5705:

CMS has been the default for a long time now (even back when LogMP was the default).  TMP
won't change things that much for the append-only case.

I think even on fast disks your merging can fall behind: it's a question of whether the indexing
threads can produce segments faster than merging can consolidate them.  Also, the amount of
free RAM that the OS can use for readahead on the files opened for merging its can have a
big impact on merge performance.

If you search "pausing thread" and "unpausing thread" you should see it pausing the largest
merge(s) when more than one are running.  Search for "too many merges; stalling..." to see
when the harsh back-pressure kicks in.

Doing all merging in the end is somewhat dangerous; you should only do it if you know you
will do no searching on the index until the merging has completed.  I suspect it will net/net
make indexing take longer because you are not soaking up concurrency during indexing to get
merges done.

Net/net it's really important that Lucene doesn't allow too many segments in the index; the
"harsh" back-pressure Lucene applies today (hard stall of ALL indexing threads) is effective
but ... harsh.

If we improved CMS to make this behavior "optional", so that by default it continued its effective-but-harsh-back-pressure,
but then an app (Solr, ES) could instead do its own thing (ES throttles down to one indexing
thread instead of 0 that Lucene does), then Solr could do something similar here.  Maybe open
a new issue for that? (Hmm: is Solr using multiple indexing threads in your case...?).

> 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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