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https://issues.apache.org/jira/browse/HADOOP-885?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12469638
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Raghu Angadi commented on HADOOP-885:
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Should we have different patch for maxIdleTime change?
The main cost of keeping connections longer is the cost of poll() and iterating
in cleanupConnections(). This cost will become more significant with larger
number of clients and datanode. I would suggest following changes
1) cleanupConnections() should be called only once in few seconds.
2) we should start using epoll() added in JDK 1.5.10 (this is a java cmd line
option) http://java.sun.com/j2se/1.5.0/ReleaseNotes.html#150_10
With these we can set timeout much higher, may be 1 minute.
> Reduce CPU usage on namenode: gettimeofday
> ------------------------------------------
>
> Key: HADOOP-885
> URL: https://issues.apache.org/jira/browse/HADOOP-885
> Project: Hadoop
> Issue Type: Bug
> Components: dfs
> Affects Versions: 0.10.1
> Reporter: dhruba borthakur
> Assigned To: dhruba borthakur
> Attachments: gettime1.patch, WallClock.java
>
>
> On a 900 node idle cluster, the namenode spends about 20% of CPU. Most of
> this CPU is spent processing pure heartbeats. No jobs are running on this
> cluster and all nodes are alive and acting well.
> Of the total namenode CPU usage, about 12% is in usermode and about 70% is in
> kernel mode! The question that natually arises is why is heartbeat processing
> taking so much time in kernel mode?
> An strace of namenode reveals that a 20 second period has about 52000
> syscalls with the following breakup:
> gettimeofday : 18000 calls
> accept : 2655 calls
> close : 2655 calls
> shutdown : 2655 calls
> fcntl : 7965 calls
> read : 7965 calls
> futex : 5295 calls
> poll : 4894 calls
> A code inspection reveals that the code is doing multiple (about 5) calls to
> System.currentTimeMillis() in processing a single request in the RPC.java and
> Server.java classes. This might mean that there is a possibility of
> optimization.
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