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Java IoT: Article

Dealing with Application Deadlocks

Detecting and reporting deadlocks with the WebLogic Server

The Jstack utility may also automatically detect deadlocks in the thread dump and display an output as follows :

Found one Java-level deadlock:

=============================
"Thread-1":
  waiting to lock monitor 0x02a6ee64 (object 0x229bd238, 
  a java.lang.Object), which is held by "Thread-0"
"Thread-0":
  waiting to lock monitor 0x02a6eecc (object 0x229bd240, 
  a java.lang.Object), which is held by "Thread-1"
 
Java stack information for the threads listed above:
===================================================
"Thread-1":
        at SimpleDeadLock$Thread2.run(SimpleDeadLock.java:37)
        - waiting to lock <0x229bd238> (a java.lang.Object)
        - locked <0x229bd240> (a java.lang.Object)
"Thread-0":
        at SimpleDeadLock$Thread1.run(SimpleDeadLock.java:24)
        - waiting to lock <0x229bd240> (a java.lang.Object)
        - locked <0x229bd238> (a java.lang.Object)
 
Found 1 deadlock.    

Once the thread dump has been acquired, you may use a thread dump analyzing tool to generate a report based on its contents.

Dealing with Deadlocks
Ultimately the deadlock must be cured by making changes to the piece of code where the threads continuously get stuck. However, since this may not be possible immediately in a production environment, the following are a few temporary measures one may take in order to deal with the issue:

  • Stuck Thread Reporting:
    WebLogic has the ability to identify and report stuck threads corresponding to a server and print the result in the logs to which the server output is redirected. A thread is diagnosed as stuck if it is continually working (not idle) for a set period of time. You may also configure certain parameters related to the stuck thread identification as follows :

Login to AdminConsole->Servers -> <server name> -> Tuning

a) configure ‘Stuck Thread Max Time'

Enter the number of seconds that a thread must be continually working before this server diagnoses the thread as being stuck. By default, WebLogic Server considers a thread to be "stuck" after 600 seconds of continuous use.

b) configure ‘Stuck Thread Timer Interval'

Enter the number of seconds after which WebLogic Server periodically scans threads to see if they have been continually working for the length of time specified by Stuck Thread Max Time. By default, WebLogic Server sets this interval to 600 seconds.

The output in the servers logs looks like the following:

<Feb 29, 2011 12:00:00 PM UTC> <Error> <WebLogicServer> <BEA-000337> <[STUCK] ExecuteThread: 'xx' for queue: 'WebLogic.kernel.Default (self-tuning)' has been busy for "xxx" seconds working on the request "xxxxxxxxxx" , which is more than the configured time (StuckThreadMaxTime) of "600" seconds. Stack trace:

An alarm may be configured to sniff the server logs at regular intervals and inform the concerned party when a certain number of stuck threads have been reported by WebLogic.

  • Stuck Thread Reaction:

At the server level:

Login to AdminConsole->Servers -> <server name> -> Overload

At the server level, the server's Overload Configuration tab controls its stuck thread behavior. ‘Max Stuck Thread Time' specifies the length of time after which the server considers a thread stuck. If ‘Stuck Thread Count' threads become stuck, the server transitions itself to a failed state. Once the server transitions to a failed state, the ‘Failure Action' parameter controls what action to take to correct the situation.

At the work manager level:
Work managers can define how they want to handle stuck threads. Work managers support defining stuck thread behavior using a shutdown trigger. A shutdown trigger tells the work manager that it should shut itself down if there are <stuck-thread-count> threads executing requests on its behalf that are stuck for longer than <max-stuck-thread-time> seconds. This work manager definition shown here tells the work manager to shutdown if 5 threads are stuck for more than 300 seconds.

<weblogic-enterprise-bean>
<ejb-name>MyEjb</ejb-name>
<dispatch-policy> MyStuckThreadWorkManager</dispatch-policy>

.................................................................................................................................................................................
</weblogic-enterprise-bean>

<work-manager>
<name>MyStuckThreadWorkManager</name>
<work-manager-shutdown-trigger>
<max-stuck-thread-time>300</max-stuck-thread-time>
<stuck-thread-count>5</stuck-thread-count>
</work-manager-shutdown-trigger>
</work-manager>

At the application level:
Enterprise applications support defining stuck thread behavior using an admin trigger. An admin trigger tells the server that it should transition the application into admin mode if there are <stuck-thread-count> threads executing requests on its behalf that are stuck for longer than <max-stuck-thread-time> seconds. One important difference about the application admin trigger is that it will automatically switch the application back from admin to running mode if the stuck thread condition clears. You define the admin trigger using the <application-admin-mode-trigger> element of the enterprise application's weblogic-application.xml deployment descriptor. The admin trigger definition shown here tells the server to put the application into admin mode if 5 threads are stuck for more than 300 seconds.

<application-admin-mode-trigger>
<max-stuck-thread-time>300</max-stuck-thread-time>
<stuck-thread-count>5</stuck-thread-count>
</application-admin-mode-trigger>

Summary
Applications freezing in the production environment on a regular basis can cause nightmares. The root cause many times can be deadlocks, which can be confirmed by analyzing the thread dump of an application that appears to be hung. The WebLogic server may be configured to detect and report these deadlocks in the logs as ‘stuck threads.' The WebLogic server also has the ability to take certain actions based on the detection of one or more stuck threads that can be configured at the server, work manager and application level.

More Stories By Sanat Vij

Sanat Vij is a professional software engineer currently working at CenturyLink. He has vast experience in developing high availability applications, configuring application servers, JVM profiling and memory management. He specializes in performance tuning of applications, reducing response times, and increasing stability.

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