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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5957) Provide support for different mmap cache retention policies in ShortCircuitCache.
Date Wed, 19 Feb 2014 02:09:21 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5957?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13905012#comment-13905012

Colin Patrick McCabe commented on HDFS-5957:

bq. This usage pattern in combination with zero-copy read causes retention of a large number
of memory-mapped regions in the ShortCircuitCache. Eventually, YARN's resource check kills
the container process for exceeding the enforced physical memory bounds.

mmap regions don't consume physical memory.  They do consume virtual memory.

I don't think YARN should limit the consumption of virtual memory.  virtual memory imposes
almost no cost on the system and limiting it leads to problems like this one.

It should be possible to limit the consumption of actual memory (not virtual address space)
and solve this problem that way.  What do you think?

bq. As a workaround, I advised Gopal to downtune dfs.client.mmap.cache.timeout.ms to make
the munmap happen more quickly. A better solution would be to provide support in the HDFS
client for a caching policy that fits this usage pattern.

In our tests, mmap provided no performance advantage unless it was reused.  If Gopal needs
to purge mmaps immediately after using them, the correct thing is simply not to use zero-copy

> 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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