Sean Chow created HDFS-14536:
--------------------------------
Summary: 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: 2.7.0, 2.6.0
Reporter: Sean Chow
I found all my my request blocked in every 6hours 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}
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
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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