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From "Suresh Srinivas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2421) Improve the concurrency of SerialNumberMap in NameNode
Date Wed, 15 Aug 2012 02:18:38 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2421?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13434756#comment-13434756
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Suresh Srinivas commented on HDFS-2421:
---------------------------------------

Jing, given the performance numbers you have posted for the map is being used in a tight loop,
I doubt if this makes such a big difference for the overall NameNode performance.

Hairong or Weiyan, do you have any numbers to substantiate the improvement? BTW we should
create a separate jira to improve the get method which checks the map entries two times, once
with contains then to get.
                
> 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: HDFS-2421.patch, 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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