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https://issues.apache.org/jira/browse/HDFS-14536?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16880466#comment-16880466
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Sean Chow commented on HDFS-14536:
----------------------------------
This patch remove {{ynchronized(dataset)}} from {{scan()}}, and it doesn't
broke the inconsistent check and update result.
With synchronized:
!reconcile-with-synchronized.png!
After removing the synchronized:
!reconcile-with-removing-synchronized.png!
> It's safe to remove synchronized from reconcile scan
> ----------------------------------------------------
>
> Key: HDFS-14536
> URL: https://issues.apache.org/jira/browse/HDFS-14536
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 2.6.0, 2.7.0
> Reporter: Sean Chow
> Priority: Major
> Attachments: HDFS-14536.00.patch,
> reconcile-with-removing-synchronized.png, reconcile-with-synchronized.png
>
>
> I found all my my request blocked in every 6 hours and use jstack to dump
> threads:
> {code:java}
> "1490900528@qtp-341796579-35" #132086969 daemon prio=5 os_prio=0
> tid=0x00007f7ed01c3800 nid=0x1a1c waiting for monitor entry
> [0x00007f7e9e42b000]
> java.lang.Thread.State: BLOCKED (on object monitor)
> at
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsVolumeImpl.getDfsUsed(FsVolumeImpl.java:295)
> - waiting to lock <0x00000002c00410f8> (a
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl)
> at
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl.getVolumeInfo(FsDatasetImpl.java:2323)
> /// lots of BLOCKED for about one minute
> "java.util.concurrent.ThreadPoolExecutor$Worker@49d8fff9[State = -1, empty
> queue]" #77 daemon prio=5 os_prio=0 tid=0x00007f7ed8590800 nid=0x49cc
> runnable [0x00007f7ea0344000]
> java.lang.Thread.State: RUNNABLE
> at
> org.apache.hadoop.hdfs.server.datanode.DatanodeUtil.idToBlockDir(DatanodeUtil.java:120)
> at
> org.apache.hadoop.hdfs.server.datanode.ReplicaInfo.getDir(ReplicaInfo.java:143)
> at
> org.apache.hadoop.hdfs.server.datanode.ReplicaInfo.<init>(ReplicaInfo.java:95)
> at
> org.apache.hadoop.hdfs.server.datanode.FinalizedReplica.<init>(FinalizedReplica.java:60)
> at
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl.getFinalizedBlocks(FsDatasetImpl.java:1510)
> - locked <0x00000002c00410f8> (a
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl)
> at
> org.apache.hadoop.hdfs.server.datanode.DirectoryScanner.scan(DirectoryScanner.java:437)
> - locked <0x00000002c00410f8> (a
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl)
> at
> org.apache.hadoop.hdfs.server.datanode.DirectoryScanner.reconcile(DirectoryScanner.java:404)
> at
> org.apache.hadoop.hdfs.server.datanode.DirectoryScanner.run(DirectoryScanner.java:360)
> at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
> at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
> at
> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
> at
> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
> at
> java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
> at
> java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
> at java.lang.Thread.run(Thread.java:745)
> {code}
> This problem may relate to lots of blocks in every datanode.
> After reviewing the DirectoryScanner behavior, I think the lock
> {{synchronized(dataset)}} in {{scan()}} can be removed safely.
> As dataset.getFinalizedBlocks() is a synchronized method, the memReport will
> be generated from volumeMap with lock. After that , the outside lock
> {{synchronized(dataset)}} is needless but I've encounter the later while-loop
> running 20 seconds that blocking other threads.
> Yes, other blocks could be added or deleted from memory or disk, but it has
> no affect to get the differences between them. Because the later fixing
> processing will handle both sides.
>
> {code:java}
> void scan() {
> clear();
> Map<String, ScanInfo[]> diskReport = getDiskReport();
> // Hold FSDataset lock to prevent further changes to the block map
> synchronized(dataset) { // remove synchronized?
> for (Entry<String, ScanInfo[]> entry : diskReport.entrySet()) {
> String bpid = entry.getKey();
> ScanInfo[] blockpoolReport = entry.getValue();
> Stats statsRecord = new Stats(bpid);
> stats.put(bpid, statsRecord);
> LinkedList<ScanInfo> diffRecord = new LinkedList<ScanInfo>();
> diffs.put(bpid, diffRecord);
> statsRecord.totalBlocks = blockpoolReport.length;
> // getFinalizedBlocks is synchronized
> List<FinalizedReplica> bl = dataset.getFinalizedBlocks(bpid);
> FinalizedReplica[] memReport = bl.toArray(new
> FinalizedReplica[bl.size()]);
> Arrays.sort(memReport); // Sort based on blockId
> int d = 0; // index for blockpoolReport
> int m = 0; // index for memReprot
> while (m < memReport.length && d < blockpoolReport.length) {
> // may take 20s to finish this loop
> }
> } //end for
> } //end synchronized
> }{code}
>
> Let say block A is added and block B is deleted after
> {{dataset.getFinalizedBlocks(bpid)}} called. That means the disk file and
> memory is updated. Because we have a memory finalized blocks snapshot before,
> and just care about those blocks consistence.
> If the difference report occasionally has that deleted block B, the later
> {{checkAndUpdate}} will handle it in the right way.
> I've compile this patch and try will test it in my enviroment.
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