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https://issues.apache.org/jira/browse/HDFS-8429?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14551311#comment-14551311
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Colin Patrick McCabe commented on HDFS-8429:
--------------------------------------------
It's very surprising that you managed to run out of memory there. The number
of readable file descriptors is unlikely to be more than a few thousand at
most, and therefore you failed to allocate a < 4kb memory region. Either that,
or you somehow got a very large number of readable file descriptors all at once
for some reason. We could cache this memory to avoid doing repeated
malloc/free cycles, but I am surprised that this is an issue. I suspect that
even if this is fixed, you will have many other unrelated issues that will make
the system unusable if memory is truly exhausted.
{code}
JNIEXPORT jobject JNICALL
Java_org_apache_hadoop_net_unix_DomainSocketWatcher_00024FdSet_getAndClearReadableFds(
JNIEnv *env, jobject obj)
{
...
done:
free(carr);
if (jthr) {
(*env)->DeleteLocalRef(env, jarr);
jarr = NULL;
}
return jarr;
}
{code}
This should be calling {{(*env)->Throw(env, jthr)}}
> Death of watcherThread making other local read blocked
> ------------------------------------------------------
>
> Key: HDFS-8429
> URL: https://issues.apache.org/jira/browse/HDFS-8429
> Project: Hadoop HDFS
> Issue Type: Bug
> Affects Versions: 2.6.0
> Reporter: zhouyingchao
> Assignee: zhouyingchao
>
> 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 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$2.run(DomainSocketWatcher.java:463)
> at java.lang.Thread.run(Thread.java:662)
> {code}
> The line 463 is following code snippet:
> {code}
> try {
> for (int fd : fdSet.getAndClearReadableFds()) {
> sendCallbackAndRemove("getAndClearReadableFds", entries, fdSet,
> fd);
> }
> {code}
> getAndClearReadableFds is a native method which will malloc an int array.
> Since our memory is very tight, it looks like the malloc failed and a NULL
> pointer is returned.
> The bad thing is that other threads then blocked in stack like this:
> {code}
> "DataXceiver for client
> unix:/home/work/app/hdfs/c3prc-micloud/datanode/dn_socket [Waiting for
> operation #1]" daemon prio=10 tid=0x00007f0c9c086d90 nid=0x8fc3 waiting on
> condition [0x00007f09b9856000]
> java.lang.Thread.State: WAITING (parking)
> at sun.misc.Unsafe.park(Native Method)
> - parking to wait for <0x00000007b0174808> (a
> java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
> at java.util.concurrent.locks.LockSupport.park(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)
> {code}
> IMO, we should exit the DN so that the users can know that something go
> wrong and fix it.
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