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From "Tsz Wo (Nicholas), SZE (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2421) Improve the concurrency of SerialNumberMap in NameNode
Date Wed, 15 Aug 2012 08:16:37 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2421?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13434877#comment-13434877
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Tsz Wo (Nicholas), SZE commented on HDFS-2421:
----------------------------------------------

Hi Jing, I think we cannot simply remove "synchronized" from get(T t).  Otherwise, there are
race conditions in the following code.
{code}
      if (sn == null) {
        sn = nextSerialNumber();
        t2i.put(t, sn);
        i2t.put(sn, t);
      }
{code}
                
> 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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