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From "Arpit Agarwal (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9668) Optimize the locking in FsDatasetImpl
Date Tue, 29 Nov 2016 19:16:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9668?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15706254#comment-15706254
] 

Arpit Agarwal commented on HDFS-9668:
-------------------------------------

A few early comments on the v25 patch:
# DirectoryScanner.java:391: There are no changes to the dataset state or on-disk files here
so the read-lock will suffice. Did I miss any state changes?
# FsVolumeImpl.java:340: Not sure about this one. The documentation of {{BlockPoolSlice#decDfsUsed}}
states _It must be synchronized from caller._
# ReplicaMap.java:39: Can we postpone changing ReplicaMap to another Jira and keep this Jira
for the FsDatasetImpl change only?

Note that [ReentrantReadWriteLock|https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/ReentrantReadWriteLock.html]
does not support upgrading, i.e. getting the write lock while holding the read lock. I'll
watch out for this while reviewing but it would be good to have a second review from this
perspective so we don't introduce a deadlock.

> Optimize the locking in FsDatasetImpl
> -------------------------------------
>
>                 Key: HDFS-9668
>                 URL: https://issues.apache.org/jira/browse/HDFS-9668
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: datanode
>            Reporter: Jingcheng Du
>            Assignee: Jingcheng Du
>         Attachments: HDFS-9668-1.patch, HDFS-9668-10.patch, HDFS-9668-11.patch, HDFS-9668-12.patch,
HDFS-9668-13.patch, HDFS-9668-14.patch, HDFS-9668-14.patch, HDFS-9668-15.patch, HDFS-9668-16.patch,
HDFS-9668-17.patch, HDFS-9668-18.patch, HDFS-9668-19.patch, HDFS-9668-19.patch, HDFS-9668-2.patch,
HDFS-9668-20.patch, HDFS-9668-21.patch, HDFS-9668-22.patch, HDFS-9668-23.patch, HDFS-9668-23.patch,
HDFS-9668-24.patch, HDFS-9668-25.patch, HDFS-9668-3.patch, HDFS-9668-4.patch, HDFS-9668-5.patch,
HDFS-9668-6.patch, HDFS-9668-7.patch, HDFS-9668-8.patch, HDFS-9668-9.patch, execution_time.png
>
>
> During the HBase test on a tiered storage of HDFS (WAL is stored in SSD/RAMDISK, and
all other files are stored in HDD), we observe many long-time BLOCKED threads on FsDatasetImpl
in DataNode. The following is part of the jstack result:
> {noformat}
> "DataXceiver for client DFSClient_NONMAPREDUCE_-1626037897_1 at /192.168.50.16:48521
[Receiving block BP-1042877462-192.168.50.13-1446173170517:blk_1073779272_40852]" - Thread
t@93336
>    java.lang.Thread.State: BLOCKED
> 	at org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl.createRbw(FsDatasetImpl.java:1111)
> 	- waiting to lock <18324c9> (a org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl)
owned by "DataXceiver for client DFSClient_NONMAPREDUCE_-1626037897_1 at /192.168.50.16:48520
[Receiving block BP-1042877462-192.168.50.13-1446173170517:blk_1073779271_40851]" t@93335
> 	at org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl.createRbw(FsDatasetImpl.java:113)
> 	at org.apache.hadoop.hdfs.server.datanode.BlockReceiver.<init>(BlockReceiver.java:183)
> 	at org.apache.hadoop.hdfs.server.datanode.DataXceiver.writeBlock(DataXceiver.java:615)
> 	at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.opWriteBlock(Receiver.java:137)
> 	at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.processOp(Receiver.java:74)
> 	at org.apache.hadoop.hdfs.server.datanode.DataXceiver.run(DataXceiver.java:235)
> 	at java.lang.Thread.run(Thread.java:745)
>    Locked ownable synchronizers:
> 	- None
> 	
> "DataXceiver for client DFSClient_NONMAPREDUCE_-1626037897_1 at /192.168.50.16:48520
[Receiving block BP-1042877462-192.168.50.13-1446173170517:blk_1073779271_40851]" - Thread
t@93335
>    java.lang.Thread.State: RUNNABLE
> 	at java.io.UnixFileSystem.createFileExclusively(Native Method)
> 	at java.io.File.createNewFile(File.java:1012)
> 	at org.apache.hadoop.hdfs.server.datanode.DatanodeUtil.createTmpFile(DatanodeUtil.java:66)
> 	at org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.BlockPoolSlice.createRbwFile(BlockPoolSlice.java:271)
> 	at org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsVolumeImpl.createRbwFile(FsVolumeImpl.java:286)
> 	at org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl.createRbw(FsDatasetImpl.java:1140)
> 	- locked <18324c9> (a org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl)
> 	at org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl.createRbw(FsDatasetImpl.java:113)
> 	at org.apache.hadoop.hdfs.server.datanode.BlockReceiver.<init>(BlockReceiver.java:183)
> 	at org.apache.hadoop.hdfs.server.datanode.DataXceiver.writeBlock(DataXceiver.java:615)
> 	at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.opWriteBlock(Receiver.java:137)
> 	at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.processOp(Receiver.java:74)
> 	at org.apache.hadoop.hdfs.server.datanode.DataXceiver.run(DataXceiver.java:235)
> 	at java.lang.Thread.run(Thread.java:745)
>    Locked ownable synchronizers:
> 	- None
> {noformat}
> We measured the execution of some operations in FsDatasetImpl during the test. Here following
is the result.
> !execution_time.png!
> The operations of finalizeBlock, addBlock and createRbw on HDD in a heavy load take a
really long time.
> It means one slow operation of finalizeBlock, addBlock and createRbw in a slow storage
can block all the other same operations in the same DataNode, especially in HBase when many
wal/flusher/compactor are configured.
> We need a finer grained lock mechanism in a new FsDatasetImpl implementation and users
can choose the implementation by configuring "dfs.datanode.fsdataset.factory" in DataNode.
> We can implement the lock by either storage level or block-level.



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