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From Filip Hanik - Dev Lists <devli...@hanik.com>
Subject Re: tcp and nio transport considerations
Date Fri, 19 Sep 2008 04:17:41 GMT
it might help setting transport.soTimeout and that should cause the 
socketWrite to back out, if it gets stuck.
what you're explaining is a common problem with many AMQ installations, 
and yes, the correct way to fix it would be to remove the locks that are 
in the stack trace

Filip

Manuel Teira Paz wrote:
> Filip Hanik - Dev Lists escribió:
>> hi Manuel,
>> I may not be understanding your theory completely, but if I do, I'd have
>> to disagree with parts of your assessment,
>>
>> the problem you describe doesn't really have anything to do with
>> blocking vs non blocking IO. instead its the implementation on top of
>> the socket API.
>>
>> taking a simple java program, you can read and write from blocking
>> sockets simultaneously.
>>   
> Hello Filip, and thanks for your comments. Actually, yes, you must be 
> able to read and write simultaneously on a given socket. Sorry for 
> being to clear enough in my exposition. I've taken a deeper look into 
> the details of the problem and the actual problem is that all my 
> consumer threads get locked, one trying to write in the socket, and 
> the others trying to adquire the MutexTransport in the transport 
> filter chain. What they are actually trying to do is to ack some 
> already sent messages. The stack looks like this:
>
> "Session(recv,TaskManagerQueue)#56" prio=10 tid=0x00b4a6d8 nid=0x6a 
> runnable [0xeb9fe000..0xeb9ffaa8]at 
> java.net.SocketOutputStream.socketWrite0(Native Method)
> at java.net.SocketOutputStream.socketWrite(Unknown Source)
> at java.net.SocketOutputStream.write(Unknown Source)
> at 
> org.apache.activemq.transport.tcp.TcpBufferedOutputStream.flush(TcpBufferedOutputStream.java:109)

>
> at java.io.DataOutputStream.flush(Unknown Source)
> at 
> org.apache.activemq.transport.tcp.TcpTransport.oneway(TcpTransport.java:119) 
>
> at 
> org.apache.activemq.transport.InactivityMonitor.oneway(InactivityMonitor.java:145) 
>
> at 
> org.apache.activemq.transport.TransportFilter.oneway(TransportFilter.java:80) 
>
> at 
> org.apache.activemq.transport.WireFormatNegotiator.oneway(WireFormatNegotiator.java:93)

>
> at 
> org.apache.activemq.transport.failover.FailoverTransport.oneway(FailoverTransport.java:392)

>
> - locked <0x2abab308> (a java.lang.Object)
> at 
> org.apache.activemq.transport.MutexTransport.oneway(MutexTransport.java:47) 
>
> - locked <0x2aba90f0> (a java.lang.Object)
> at 
> org.apache.activemq.transport.ResponseCorrelator.oneway(ResponseCorrelator.java:60) 
>
> at 
> org.apache.activemq.ActiveMQConnection.asyncSendPacket(ActiveMQConnection.java:1155)

>
> at 
> org.apache.activemq.TransactionContext.begin(TransactionContext.java:201)
> at 
> org.apache.activemq.ActiveMQSession.doStartTransaction(ActiveMQSession.java:1564) 
>
> at 
> org.apache.activemq.ActiveMQMessageConsumer.ackLater(ActiveMQMessageConsumer.java:699)

>
> at 
> org.apache.activemq.ActiveMQMessageConsumer.beforeMessageIsConsumed(ActiveMQMessageConsumer.java:651)

>
> at 
> org.apache.activemq.ActiveMQMessageConsumer.receive(ActiveMQMessageConsumer.java:487)

>
>
> That is the locker, and never goes out of 
> java.net.SocketOutputStream.socketWrite0. What I understand from that 
> stack is that is trying to ack some messages before actually consuming 
> the message, so, sending through the socket is involved. The thread is 
> runnable, but since the socket buffer is full, is not able to continue.
>
> The other consumers sharing that connection show this stack:
>
> waiting for monitor entry [0xec0ff000..0xec0ffa28]
> at 
> org.apache.activemq.transport.MutexTransport.oneway(MutexTransport.java:46) 
>
> - waiting to lock <0x2aba90f0> (a java.lang.Object)
> at 
> org.apache.activemq.transport.ResponseCorrelator.oneway(ResponseCorrelator.java:60) 
>
> at 
> org.apache.activemq.ActiveMQConnection.asyncSendPacket(ActiveMQConnection.java:1155)

>
> at 
> org.apache.activemq.TransactionContext.begin(TransactionContext.java:201)
> at 
> org.apache.activemq.ActiveMQSession.doStartTransaction(ActiveMQSession.java:1564) 
>
> at 
> org.apache.activemq.ActiveMQMessageConsumer.ackLater(ActiveMQMessageConsumer.java:699)

>
> at 
> org.apache.activemq.ActiveMQMessageConsumer.beforeMessageIsConsumed(ActiveMQMessageConsumer.java:651)

>
> at 
> org.apache.activemq.ActiveMQMessageConsumer.receive(ActiveMQMessageConsumer.java:487)

>
>
> In this situation, nobody is able to consume from the socket, since 
> all the consumers are locked trying to ack some messages. So, it seems 
> to me a implicit deadlock.
>
> On the other way, ActiveMQ Transport, (I think they are responsible 
> for reading stuff from the socket)  threads seems to be stuck in a 
> similar situation (this is from a different stack dump):
>
> "ActiveMQ Transport: tcp:///127.0.0.1:25047" daemon prio=10 
> tid=0x003764c0 nid=0x30 runnable [0xef3fe000..0xef3ffca8]
> at java.net.SocketOutputStream.socketWrite0(Native Method)
> at java.net.SocketOutputStream.socketWrite(Unknown Source)
> at java.net.SocketOutputStream.write(Unknown Source)
> at 
> org.apache.activemq.transport.tcp.TcpBufferedOutputStream.flush(TcpBufferedOutputStream.java:109)

>
> at java.io.DataOutputStream.flush(Unknown Source)
> at 
> org.apache.activemq.transport.tcp.TcpTransport.oneway(TcpTransport.java:119) 
>
> at 
> org.apache.activemq.transport.InactivityMonitor.oneway(InactivityMonitor.java:145) 
>
> at 
> org.apache.activemq.transport.TransportFilter.oneway(TransportFilter.java:80) 
>
> at 
> org.apache.activemq.transport.WireFormatNegotiator.oneway(WireFormatNegotiator.java:93)

>
> at 
> org.apache.activemq.transport.MutexTransport.oneway(MutexTransport.java:47) 
>
> - locked <0x2ab72b20> (a java.lang.Object)
> at 
> org.apache.activemq.broker.TransportConnection.dispatch(TransportConnection.java:1138)

>
> at 
> org.apache.activemq.broker.TransportConnection.processDispatch(TransportConnection.java:805)

>
> at 
> org.apache.activemq.broker.TransportConnection.dispatchSync(TransportConnection.java:770)

>
> at 
> org.apache.activemq.broker.region.PrefetchSubscription.dispatch(PrefetchSubscription.java:404)

>
> at 
> org.apache.activemq.broker.region.QueueSubscription.dispatch(QueueSubscription.java:172)

>
> at 
> org.apache.activemq.broker.region.PrefetchSubscription.dispatchMatched(PrefetchSubscription.java:369)

>
> at 
> org.apache.activemq.broker.region.PrefetchSubscription.acknowledge(PrefetchSubscription.java:204)

>
> - locked <0x2af14b48> (a 
> org.apache.activemq.broker.region.QueueSubscription)
> at 
> org.apache.activemq.broker.region.AbstractRegion.acknowledge(AbstractRegion.java:299)

>
> at 
> org.apache.activemq.broker.region.RegionBroker.acknowledge(RegionBroker.java:402) 
>
> at 
> org.apache.activemq.broker.TransactionBroker.acknowledge(TransactionBroker.java:177)

>
> at 
> org.apache.activemq.broker.BrokerFilter.acknowledge(BrokerFilter.java:74)
> at 
> org.apache.activemq.broker.BrokerFilter.acknowledge(BrokerFilter.java:74)
> at 
> org.apache.activemq.broker.MutableBrokerFilter.acknowledge(MutableBrokerFilter.java:88)

>
> at 
> org.apache.activemq.broker.TransportConnection.processMessageAck(TransportConnection.java:506)

>
> at org.apache.activemq.command.MessageAck.visit(MessageAck.java:179)
> at 
> org.apache.activemq.broker.TransportConnection.service(TransportConnection.java:294)

>
> at 
> org.apache.activemq.broker.TransportConnection$1.onCommand(TransportConnection.java:185)

>
> at 
> org.apache.activemq.transport.TransportFilter.onCommand(TransportFilter.java:65) 
>
> at 
> org.apache.activemq.transport.WireFormatNegotiator.onCommand(WireFormatNegotiator.java:133)

>
> at 
> org.apache.activemq.transport.InactivityMonitor.onCommand(InactivityMonitor.java:124)

>
> at 
> org.apache.activemq.transport.TransportSupport.doConsume(TransportSupport.java:84) 
>
> at 
> org.apache.activemq.transport.tcp.TcpTransport.run(TcpTransport.java:137)
> at java.lang.Thread.run(Unknown Source)
>
>
> So, will non-blocking IO be a solution for this? After a deeper look, 
> I'm afraid it won't. The only actual difference from this point of 
> view is that transport.nio.NIOOutputStream.flush() will write the data 
> in chunks, as big as the socket allows, but it won't give up until all 
> the data get written. We won't be stuck in the socketWrite0 call, 
> instead of that, the "while (remaining > 0)" loop in NIOOutputStream, 
> will be repeated forever, and since the other consumers sharing the 
> connection are also trying to write into the socket to ack some 
> message, will be no chance to free the socket buffer.
>
> If I'm not mistaken, since both "ActiveMQ Transport" and "Session" 
> threads need to write into the socket, this could lead to the 
> situation where everybody is trying to do so, finishing with some 
> threads locking into the socketWrite0 call, and some others waiting 
> for the MutexTransport writeLock to proceed.
>
> After this new review, thanks to Filip, I'm afraid that NIO is not 
> going to be a solution (unfortunately I was not able to reproduce the 
> hang in our labs). So, the questions are:
>
> -Do you agree with this new analysis?
> -Is this design still present in the 5.x releases?
>
> I'm not sure whether using different connections for consumers and 
> producers would be a valid workaround. Perhaps it will just mitigate 
> the situation, as more sockets get involved and so, more effective 
> buffer space gets used, but I would like to be sure that this is not 
> going to happen, since it leads to service unavailability situations 
> in the product.
>
> Best regards and thanks a lot for your feedback. Looking forward for 
> more. :)
>
> -
> Manuel.
>
>
>
>


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