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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5957) Provide support for different mmap cache retention policies in ShortCircuitCache.
Date Mon, 03 Mar 2014 20:28:30 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5957?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13918542#comment-13918542
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Todd Lipcon commented on HDFS-5957:
-----------------------------------

bq. That was attempted & we do follow that codepath for the remote reads. That is slower
than the zero copy read because to produce a direct byte buffer, the JVM has to defragment
memory & produce a contiguous large memory region - this triggered a full GC pass, which
caused stragglers with container reuse.

This doesn't make sense to me -- direct buffers are off-heap and allocating one is really
just a wrapper around Unsafe.allocateMemory(), which itself is just a regular old malloc.

Maybe something else was going on there?

> Provide support for different mmap cache retention policies in ShortCircuitCache.
> ---------------------------------------------------------------------------------
>
>                 Key: HDFS-5957
>                 URL: https://issues.apache.org/jira/browse/HDFS-5957
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client
>    Affects Versions: 2.3.0
>            Reporter: Chris Nauroth
>
> Currently, the {{ShortCircuitCache}} retains {{mmap}} regions for reuse by multiple reads
of the same block or by multiple threads.  The eventual {{munmap}} executes on a background
thread after an expiration period.  Some client usage patterns would prefer strict bounds
on this cache and deterministic cleanup by calling {{munmap}}.  This issue proposes additional
support for different caching policies that better fit these usage patterns.



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