Configuring the hang detection policy

The hang detection option for WebSphere® Application Server is turned on by default. You can configure a hang detection policy to accommodate your applications and environment so that potential hangs can be reported, providing earlier detection of failing servers. When a hung thread is detected, WebSphere Application Server notifies you so that you can troubleshoot the problem.

Before you begin

A common error in Java™ Platform, Enterprise Edition (Java EE) applications is a hung thread. A hung thread can result from a simple software defect (such as an infinite loop) or a more complex cause (for example, a resource deadlock). System resources, such as CPU time, might be consumed by this hung transaction when threads run unbounded code paths, such as when the code is running in an infinite loop. Alternately, a system can become unresponsive even though all resources are idle, as in a deadlock scenario. Unless a user or a monitoring tool reports the problem, the system may remain in this degraded state indefinitely.

Using the hang detection policy, you can specify a time that is too long for a unit of work to complete. The thread monitor checks all managed threads in the system (for example, web container threads and object request broker (ORB) threads). Unmanaged threads, which are threads created by applications, are not monitored. For more information read Hung threads in Java Platform, Enterprise Edition applications.

About this task

The thread hang detection option is enabled by default. To adjust the hang detection policy values, or to disable hang detection completely:

Procedure

  1. From the administrative console, click Servers > Application Servers > server_name
  2. Under Server Infrastructure, click Administration > Custom Properties
  3. Click New.
  4. Add the following properties:
    Information Description
    Name com.ibm.websphere.threadmonitor.interval
    Value The frequency, in seconds, at which managed threads in the selected application server will be interrogated.
    Default 180 seconds (three minutes)
    Information Description
    Name com.ibm.websphere.threadmonitor.threshold
    Value The length of time, in seconds, in which a thread can be active before it is considered hung. Any thread that is detected as active for longer than this length of time is reported as hung.
    Default 600 seconds (ten minutes)
    Information Description
    Name com.ibm.websphere.threadmonitor.false.alarm.threshold
    Value The number of times (T) that false alarms can occur before automatically increasing the threshold. It is possible that a thread that is reported as hung eventually completes its work, resulting in a false alarm. A large number of these events indicates that the threshhold value is too small. The hang detection facility can automatically respond to this situation: For every T false alarms, the threshold T is increased by a factor of 1.5. Set the value to zero (or less) to disable the automatic adjustment.
    Default 100
    Information Description
    Name com.ibm.websphere.threadmonitor.dump.java
    Value

    This property calls the dumpThreads function. Set to true to execute the dumpThreads function when a hung thread is detected and a WSVR0605W message is printed. The threads section of the javacore dump can be analyzed to determine what the reported thread and other related threads are doing.

    Set to an integer value in the range 1 through Integer.MAX_VALUE to cause the dumpThreads function to be executed when a hung thread is detected and a WSVR0605W message is printed. The integer value indicates the maximum number of times dumpThreads will be executed.
    Note: To turn off WSVR0605W messages, set com.ibm.websphere.threadmonitor.interval=0.

    [IBM i][AIX Solaris HP-UX Linux Windows]By default, the dumpThreads function creates a javacore dump. See the topic Dumping threads in server processes using scripting for information about how to disable the creation of this dump. See the Java Diagnostics guide for more information about the contents and implications of this dump.

    [z/OS]By default, the dumpThreads function creates a javacore dump, a Heapdump, and a TDUMP. See the topic Dumping threads in server processes using scripting for information about how to disable the creation of these dumps. See the Java Diagnostics guide for more information about the contents and implications of these dumps.

    Default false (0)
    Information Description
    Name com.ibm.websphere.threadmonitor.dump.java.track
    Value

    This property specifies the number of intervals the monitor will call the dumpThreads function while a thread remains hung.

    Set to an integer value in the range 2 through the value of com.ibm.websphere.threadmonitor.dump.java, which causes the dumpThreads function to be executed over subsequent monitor intervals in which a thread remains hung. The integer value indicates the maximum number of times dumpThreads is executed to track a hung thread. For example, a value of 2 causes the thread monitor to execute dumpThreads twice at the most for a hung thread, and once when the thread is initially detected as hung. The thread monitor can execute dumpThreadsand once again if the thread remains hung during the next thread monitor interval.

    This property can only be used in conjunction with the com.ibm.websphere.threadmonitor.dump.java property.

    Default (0) intervals
  5. Optional: If you want to monitor the activity of threads on which system alarms execute, add the following JVM generic arguments to the server settings.
    Information Description
    Name -Dcom.ibm.websphere.alarmthreadmonitor.generate.javacore
    Value Set to any value to cause a javacore dump to be created when an hung system alarm thread is detected. The threads section of the javacore dump can be analyzed to determine what the reported thread and other related threads are doing.
    Default Unset
    Information Description
    Name -Dcom.ibm.websphere.alarmthreadmonitor.checkinterval.millis
    Value The frequency, in milliseconds, at which system alarm threads are interrogated. Set the value to zero to disable system alarm hung thread detection. The maximum interval is 600000 (10 minutes).
    Default 10000 (10 seconds)
    Information Description
    Name -Dcom.ibm.websphere.alarmthreadmonitor.threshold.millis
    Value Set to any value integer between 10000 and 600000 (10 minutes). This argument is used to specify the length of time, in milliseconds, that a system alarm thread can be active before it is considered non-responsive. Any system alarm thread that is detected as inactive for longer than this length of time is reported as hung.
    Default 10000 (10 seconds)

    To add these arguments to the server settings, complete the following actions:

    1. Under Server Infrastructure on the server settings page in the administrative console, click Java and process management > Process definition.
    2. [z/OS] Select Control.
    3. Select Java virtual machine.
    4. Add the arguments to the JVM generic arguments section.
  6. Click Apply.
  7. Click OK.
  8. Save the changes.
    Make sure a file synchronization is performed before restarting the servers.
  9. Restart the Application Server for the changes to take effect.