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https://issues.apache.org/jira/browse/HADOOP-4346?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12637386#action_12637386
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Raghu Angadi commented on HADOOP-4346:
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I doubt 8K would be enough for a moderately busy datanode with out this patch.
Ultimately it depends on what goes on in blackbox of GC and phantom
references.. it might be fine for some users or some loads.
> Hadoop triggers a "soft" fd leak.
> ----------------------------------
>
> Key: HADOOP-4346
> URL: https://issues.apache.org/jira/browse/HADOOP-4346
> Project: Hadoop Core
> Issue Type: Bug
> Components: io
> Affects Versions: 0.17.0
> Reporter: Raghu Angadi
> Assignee: Raghu Angadi
> Attachments: HADOOP-4346-branch-18.patch, HADOOP-4346.patch
>
>
> Starting with Hadoop-0.17, most of the network I/O uses non-blocking NIO
> channels. Normal blocking reads and writes are handled by Hadoop and use our
> own cache of selectors. This cache suites well for Hadoop where I/O often
> occurs on many short lived threads. Number of fds consumed is proportional to
> number of threads currently blocked.
> If blocking I/O is done using java.*, Sun's implementation uses internal
> per-thread selectors. These selectors are closed using {{sun.misc.Cleaner}}.
> Looks like this cleaning is kind of like finalizers and tied to GC. This is
> pretty ill suited if we have many threads that are short lived. Until GC
> happens, number of these selectors keeps growing. Each selector consumes 3
> fds.
> Though blocking read and write are handled by Hadoop, {{connect()}} is still
> the default implementation that uses per-thread selector.
> Koji helped a lot in tracking this. Some sections from 'jmap' output and
> other info Koji collected led to this suspicion and will include that in the
> next comment.
> One solution might be to handle connect() also in Hadoop using our selectors.
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