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From "Nick Bailey" <nicholas.bai...@rackspace.com>
Subject Zookeeper EventThread and SendThread
Date Thu, 20 May 2010 18:30:05 GMT
Hey guys,

Question regarding zookeeper's EventThread and SendThread. I'm not quite sure what these are
used for but a stacktrace of our client application contains lines similar to

"pool-2-thread-20-EventThread" daemon prio=10 tid=0x00002aac3cb29c00 nid=0x75d waiting on
condition [0x000000006b080000..0x000000006b080b10]
   java.lang.Thread.State: WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        - parking to wait for  <0x00002aab1f577250> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:1925)
        at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:358)
        at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:414)

"pool-2-thread-20-SendThread" daemon prio=10 tid=0x00002aac3c35d400 nid=0x75c runnable [0x0000000070ede000..0x0000000070edeb90]
   java.lang.Thread.State: RUNNABLE
        at sun.nio.ch.EPollArrayWrapper.epollWait(Native Method)
        at sun.nio.ch.EPollArrayWrapper.poll(EPollArrayWrapper.java:215)
        at sun.nio.ch.EPollSelectorImpl.doSelect(EPollSelectorImpl.java:65)
        at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:69)
        - locked <0x00002aab1f571d08> (a sun.nio.ch.Util$1)
        - locked <0x00002aab1f571cf0> (a java.util.Collections$UnmodifiableSet)
        - locked <0x00002aab1f5715b8> (a sun.nio.ch.EPollSelectorImpl)
        at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:80)
        at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:921)

There are pairs of threads ranging from thread-1 to thread-50 and also multiple pairs of these
threads.  As in pool-2-thread-20-SendThread is the name of multiple threads in the trace.
 I'm debugging some load issues with our system and am suspicious that the large amount of
zookeeper threads is contributing. Would anyone be able to elaborate on the purpose of these
threads and how they are spawned?

Thanks,

Nick Bailey
Rackspace Hosting
Software Developer, Email & Apps
nicholas.bailey@rackspace.com


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