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  • 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: {code} ERROR Thread[Thread-25,5,main] terminating on unexpected exception java.lang.NullPointerException at$ at {code} The line 463 is following code snippet: {code} try { for (int fd : fdSet.getAndClearReadableFds()) { sendCallbackAndRemove("getAndClearReadableFds", entries, fdSet, fd); } {code} 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: {code} "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$ConditionObject) at java.util.concurrent.locks.LockSupport.park( at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await( at at org.apache.hadoop.hdfs.server.datanode.ShortCircuitRegistry.createNewMemorySegment( at org.apache.hadoop.hdfs.server.datanode.DataXceiver.requestShortCircuitShm( at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.opRequestShortCircuitShm( at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.processOp( at at {code} IMO, we should exit the DN so that the users can know that something go wrong and fix it.
    via by zhouyingchao,
    • java.lang.NullPointerException at$ at
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