[jira] Commented: (HADOOP-4346) Hadoop triggers a "soft" fd leak.

["Raghu Angadi (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-4346?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12637162#action_12637162 ] 

Raghu Angadi commented on HADOOP-4346:
--------------------------------------

The following shows relevant info from jmap for a datanode that had a lot fds open.

- {noformat}
#jmap with out full-GC. Includes stale objects:
# num of fds for the process : 5358

#java internal selectors
 117:          1780          42720  sun.nio.ch.Util$SelectorWrapper
 118:          1762          42288  sun.nio.ch.Util$SelectorWrapper$Closer

#Hadoop selectors
  93:          3026         121040  org.apache.hadoop.net.SocketIOWithTimeout$SelectorPool$SelectorInfo
 844:             1             40  org.apache.hadoop.net.SocketIOWithTimeout$SelectorPool$ProviderInfo

#Datanode threads 
  99:          2229         106992  org.apache.hadoop.dfs.DataNode$DataXceiver
{noformat}

- {noformat}
#jmap -histo:live immediately after the previous. This does a full-GC before counting.
#num of fds : 5187

  64:          1759          42216  sun.nio.ch.Util$SelectorWrapper
  65:          1759          42216  sun.nio.ch.Util$SelectorWrapper$Closer

 465:             4            160  org.apache.hadoop.net.SocketIOWithTimeout$SelectorPool$SelectorInfo
 772:             1             40  org.apache.hadoop.net.SocketIOWithTimeout$SelectorPool$ProviderInfo

 422:             4            192  org.apache.hadoop.dfs.DataNode$DataXceiver
{noformat} This shows that there is no fd leak in Hadoop's selector cache. DN has 4 threads doing I/O and there are 4 selectors. But there are a lot of java internal selectors open.

- {noformat}
# 'jmap -histo:live' bout 1 minute after the previous full-GC
#num of fds : 57

# There are no SelectorWrapper objects. All of these must have been closed.

 768:             1             40  org.apache.hadoop.net.SocketIOWithTimeout$SelectorPool$SelectorInfo

 730:             1             48  org.apache.hadoop.dfs.DataNode$DataXceiver
{noformat}

I will try to reproduced this myself and try out a patch for connect(). 


> 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
>
> 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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