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https://issues.apache.org/jira/browse/HDFS-2184?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13069567#comment-13069567
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Eric Payne commented on HDFS-2184:
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Hi Eli,
I noticed a slowdown in the performance of the namenode lately (in the past
month or so) when it is under stress. I also noticed a striking increase in
reliability of the namenode under stress as well. But, there are probably
opportunities in the FSNamesystem and FSDirectory classes to improve
performance.
Thanks,
-Eric
> Revisit Namenode locking
> ------------------------
>
> Key: HDFS-2184
> URL: https://issues.apache.org/jira/browse/HDFS-2184
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: name-node
> Reporter: Eli Collins
>
> While working on HDFS-988 I noticed that the locking in FSNamesystem and
> FSDirectory could be improved. Some observations:
> The namesystem lock (fsLock) is always taken before acquiring the directory
> lock (dirLock). Therefore the only time when the directory lock is needed is
> when the fsLock is taken for reading and the directory lock is taken for
> writing, but I don't think that ever happens. Therefore we can probably get
> rid of the directory lock.
> In HDFS-988 I modified handleHeartbeat to take the read lock so it's
> synchronized with register datanode. I also added a missing synchronization
> of datanodeMap to wipeDatanode because handleHeartbeat calls getDatanode()
> while only holding locks on heartbeats and datanodeMap, but registerDatanode
> mutates datanodeMap without locking either. We should revisit which
> locks/synchronization protect which data structures, there may be other
> similar bugs and also opportunities to increase parallelism.
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