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https://issues.apache.org/jira/browse/HDFS-2421?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13435628#comment-13435628
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Jing Zhao commented on HDFS-2421:
---------------------------------
Nicholas, my previous patch was just for a simple performance testing, and did
not consider too much about synchronization semantic. I guess you may want to
review Weiyan's code at
https://github.com/facebook/hadoop-20/blob/develop/src/hdfs/org/apache/hadoop/hdfs/server/namenode/SerialNumberManager.java,
and I think it should be fine in synchronization and can improve the read
performance (although the value of serial number is not increasing in the
original way).
> Improve the concurrency of SerialNumberMap in NameNode
> -------------------------------------------------------
>
> Key: HDFS-2421
> URL: https://issues.apache.org/jira/browse/HDFS-2421
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: name-node
> Reporter: Hairong Kuang
> Assignee: Weiyan Wang
> Fix For: 0.24.0
>
> Attachments: performance_100000_op.png, performance_10000_op.png,
> performance_SerialNumberManager.png, TestSerialNumberManager.java
>
>
> After enabled permission checking in our HDFS test cluster, our benchmark
> observed a significant reduced concurrency in NameNode. Investigation showed
> that most threads were blocked at acquiring the lock of
> org.apache.hadoop.hdfs.server.namenode.SerialNumberManager$SerialNumberMap.
> We used concurrentHashMap to replace Hashmap + synchronized methods, which
> greatly improved the situation.
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