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https://issues.apache.org/jira/browse/SOLR-7319?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14386834#comment-14386834
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Shawn Heisey commented on SOLR-7319:
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Good questions, [~jim.ferenczi]. The option does appear to have helped GC
pauses times for me, although it's hard to quantify. I know that the *average*
GC pause time dropped from .10 sec to .06 sec. This isn't a lot, but when
there are thousands of collections, even a small difference like that adds up.
I wish I had a way to gather median, 75th, 95th, and 99th percentile info on GC
pauses.
If you know something about how Lucene writes to disk that says it's not mmap
when the directory is mmap, then you know more than I do. I wonder whether
heavy mmap *reads* might interfere with writing to the stats file.
> Workaround the "Four Month Bug" causing GC pause problems
> ---------------------------------------------------------
>
> Key: SOLR-7319
> URL: https://issues.apache.org/jira/browse/SOLR-7319
> Project: Solr
> Issue Type: Bug
> Components: scripts and tools
> Affects Versions: 5.0
> Reporter: Shawn Heisey
> Assignee: Shawn Heisey
> Fix For: 5.1
>
> Attachments: SOLR-7319.patch, SOLR-7319.patch, SOLR-7319.patch
>
>
> A twitter engineer found a bug in the JVM that contributes to GC pause
> problems:
> http://www.evanjones.ca/jvm-mmap-pause.html
> Problem summary (in case the blog post disappears): The JVM calculates
> statistics on things like garbage collection and writes them to a file in the
> temp directory using MMAP. If there is a lot of other MMAP write activity,
> which is precisely how Lucene accomplishes indexing and merging, it can
> result in a GC pause because the mmap write to the temp file is delayed.
> We should implement the workaround in the solr start scripts (disable
> creation of the mmap statistics tempfile) and document the impact in
> CHANGES.txt.
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