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https://issues.apache.org/jira/browse/HADOOP-1269?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12489890
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Doug Cutting commented on HADOOP-1269:
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Won't different methods need to synchronize on different subsets of the
namenode's data? If so, then synchronizing on them separately must be done
very carefully to avoid deadlocks. Also, to the degree these subsets overlap,
this will provide less speedup. Finally, the two most CPU-intensive methods
you mention need the same data structures, so finer-grained synchronization
won't help them, will it?
For these reasons, my instinct would be to instead try to optimize these
methods, both so they use less CPU and also, if possible, minimizing the time
they need to keep things synchronized. Can much of their work be done
unsynchronized, with only a few critical peeks and pokes synchronized?
But my instinct could be wrong...
> DFS Scalability: namenode throughput impacted becuase of global FSNamesystem
> lock
> ---------------------------------------------------------------------------------
>
> Key: HADOOP-1269
> URL: https://issues.apache.org/jira/browse/HADOOP-1269
> Project: Hadoop
> Issue Type: Bug
> Components: dfs
> Reporter: dhruba borthakur
> Assigned To: dhruba borthakur
> Attachments: serverThreads1.html, serverThreads40.html
>
>
> I have been running a 2000 node cluster and measuring namenode performance.
> There are quite a few "Calls dropped" messages in the namenode log. The
> namenode machine has 4 CPUs and each CPU is about 30% busy. Profiling the
> namenode shows that the methods the consume CPU the most are addStoredBlock()
> and getAdditionalBlock(). The first method in invoked when a datanode
> confirms the presence of a newly created block. The second method in invoked
> when a DFSClient request a new block for a file.
> I am attaching two files that were generated by the profiler.
> serverThreads40.html captures the scenario when the namenode had 40 server
> handler threads. serverThreads1.html is with 1 server handler thread (with a
> max_queue_size of 4000).
> In the case when there are 40 handler threads, the total elapsed time taken
> by FSNamesystem.getAdditionalBlock() is 1957 seconds whereas the methods
> that that it invokes (chooseTarget) takes only about 97 seconds.
> FSNamesystem.getAdditionalBlock is blocked on the global FSNamesystem lock
> for all those 1860 seconds.
> My proposal is to implement a finer grain locking model in the namenode. The
> FSNamesystem has a few important data structures, e.g. blocksMap,
> datanodeMap, leases, neededReplication, pendingCreates, heartbeats, etc. Many
> of these data structures already have their own lock. My proposal is to have
> a lock for each one of these data structures. The individual lock will
> protect the integrity of the contents of the data structure that it protects.
> The global FSNamesystem lock is still needed to maintain consistency across
> different data structures.
> If we implement the above proposal, both addStoredBlock() and
> getAdditionalBlock() does not need to hold the global FSNamesystem lock.
> startFile() and closeFile() still needs to acquire the global FSNamesystem
> lock because it needs to ensure consistency across multiple data structures.
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