[HDFS-8429] Avoid stuck threads if there is an error in DomainSocketWatcher that stops the thread

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.6.0
• Fix Version/s: 2.8.0, 3.0.0-alpha1
• Target Version/s: 2.8.0
• Assignee: zhouyingchao
• Reporter: zhouyingchao
• Created: 19/May/15 11:26
• Updated: 30/Aug/16 01:34
• Resolved: 28/May/15 18:53

Description

In our cluster, an application is hung when doing a short circuit read of local hdfs block. By looking into the log, we found the DataNode's DomainSocketWatcher.watcherThread has exited with following log:

ERROR org.apache.hadoop.net.unix.DomainSocketWatcher: Thread[Thread-25,5,main] terminating on unexpected exception
java.lang.NullPointerException
        at org.apache.hadoop.net.unix.DomainSocketWatcher$2.run(DomainSocketWatcher.java:463)
        at java.lang.Thread.run(Thread.java:662)

The line 463 is following code snippet:

         try {
            for (int fd : fdSet.getAndClearReadableFds()) {
              sendCallbackAndRemove("getAndClearReadableFds", entries, fdSet,
                fd);
            }

getAndClearReadableFds is a native method which will malloc an int array. Since our memory is very tight, it looks like the malloc failed and a NULL pointer is returned.

The bad thing is that other threads then blocked in stack like this:

"DataXceiver for client unix:/home/work/app/hdfs/c3prc-micloud/datanode/dn_socket [Waiting for operation #1]" daemon prio=10 tid=0x00007f0c9c086d90 nid=0x8fc3 waiting on condition [0x00007f09b9856000]
   java.lang.Thread.State: WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        - parking to wait for  <0x00000007b0174808> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:156)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:1987)
        at org.apache.hadoop.net.unix.DomainSocketWatcher.add(DomainSocketWatcher.java:323)
        at org.apache.hadoop.hdfs.server.datanode.ShortCircuitRegistry.createNewMemorySegment(ShortCircuitRegistry.java:322)
        at org.apache.hadoop.hdfs.server.datanode.DataXceiver.requestShortCircuitShm(DataXceiver.java:403)
        at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.opRequestShortCircuitShm(Receiver.java:214)
        at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.processOp(Receiver.java:95)
        at org.apache.hadoop.hdfs.server.datanode.DataXceiver.run(DataXceiver.java:235)
        at java.lang.Thread.run(Thread.java:662)

IMO, we should exit the DN so that the users can know that something go wrong and fix it.