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https://issues.apache.org/jira/browse/HBASE-7295?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13528331#comment-13528331
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Lars Hofhansl commented on HBASE-7295:
--------------------------------------
Thanks Varun.
Double checked locking only works correctly if the checked object/value is
declared volatile. Otherwise the JVM is free (1) to reorder the statements and
(2) not provide the latest value to the current thread. This describes it
reasonably well
http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html (you
probably saw this, just in case)
I agree it'll make it slower again, need to performance test.
> Contention in HBaseClient.getConnection
> ---------------------------------------
>
> Key: HBASE-7295
> URL: https://issues.apache.org/jira/browse/HBASE-7295
> Project: HBase
> Issue Type: Improvement
> Affects Versions: 0.94.3
> Reporter: Varun Sharma
> Assignee: Varun Sharma
> Fix For: 0.96.0, 0.94.4
>
> Attachments: 7295-0.94.txt, 7295-0.94-v2.txt, 7295-0.94-v3.txt,
> 7295-0.94-v4.txt, 7295-trunk.txt, 7295-trunk.txt, 7295-trunk-v2.txt
>
>
> HBaseClient.getConnection() synchronizes on the connections object. We found
> severe contention on a thrift gateway which was fanning out roughly 3000+
> calls per second to hbase region servers. The thrift gateway had 2000+
> threads for handling incoming connections. Threads were blocked on the
> syncrhonized block - we set ipc.pool.size to 200. Since we are using
> RoundRobin/ThreadLocal pool only - its not necessary to synchronize on
> connections - it might lead to cases where we might go slightly over the
> ipc.max.pool.size() but the additional connections would timeout after
> maxIdleTime - underlying PoolMap connections object is thread safe.
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