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From David Sitsky <s...@nuix.com>
Subject Contention in Queue.lock() via IndirectMessageReference.lock()
Date Fri, 08 Feb 2008 16:09:44 GMT
In my application, I have a large number of threads in the broker, which 
are calling dispatchPending() on their subscriptions, but are unable to 
make fast progress, as can be seen in the following stack trace:

"ActiveMQ Transport: tcp:///192.168.222.75:56495" daemon prio=4 
tid=0x4be82000 n
id=0x1028 waiting for monitor entry [0x54fdf000..0x54fdfb00]
    java.lang.Thread.State: BLOCKED (on object monitor)
         at org.apache.activemq.broker.region.Queue.lock(Queue.java:166)
         - locked <0x0a60bf58> (a java.lang.Object)
         at 
org.apache.activemq.broker.region.IndirectMessageReference.lock(IndirectMessageReference.java:143)
         at 
org.apache.activemq.broker.region.QueueSubscription.canDispatch(QueueSubscription.java:119)
         at 
org.apache.activemq.broker.region.PrefetchSubscription.dispatchPending(PrefetchSubscription.java:466)
         - locked <0x0b55bd78> (a java.lang.Object)
         at 
org.apache.activemq.broker.region.PrefetchSubscription.acknowledge(PrefetchSubscription.java:347)
         at 
org.apache.activemq.broker.region.AbstractRegion.acknowledge(AbstractRegion.java:342)
         at 
org.apache.activemq.broker.region.RegionBroker.acknowledge(RegionBroker.java:461)
         at 
org.apache.activemq.broker.TransactionBroker.acknowledge(TransactionBroker.java:194)
         at 
org.apache.activemq.broker.BrokerFilter.acknowledge(BrokerFilter.java:73)
         at 
org.apache.activemq.broker.BrokerFilter.acknowledge(BrokerFilter.java:73)
         at 
org.apache.activemq.broker.MutableBrokerFilter.acknowledge(MutableBrokerFilter.java:87)
         at 
org.apache.activemq.broker.TransportConnection.processMessageAck(TransportConnection.java:440)
         at 
org.apache.activemq.command.MessageAck.visit(MessageAck.java:196)
....

I am trying to understand why IndirectMessageReference.lock() would 
require synchronisation on the entire Queue object it is associated with?

This is why the performance in my application is grinding... each time a 
prefetch/queue subscription is testing to see if they can dispatch a 
message, we are effectively serialising on the Queue object.

Any ideas on how we can fix this?  I have some ideas, but thought the 
experts are better qualified. :)

This became evident when I had 20 consumers hammering away.  With 2, 
you'd never see this.

Cheers,
David

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