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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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