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https://issues.apache.org/jira/browse/HDFS-5957?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13904932#comment-13904932
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Colin Patrick McCabe commented on HDFS-5957:
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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 limiting virtual memory usage is a particularly helpful policy,
and YARN should stop doing that if that is in fact what it is doing.
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 reads.
> 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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