[HDFS-2421] Improve the concurrency of SerialNumberMap in NameNode - ASF JIRA

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Details

Type: Improvement
Status: Closed
Priority: Major
Resolution: Fixed
Affects Version/s: None
Fix Version/s: 0.23.3, 2.0.2-alpha
Component/s: namenode
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Description

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.