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I have a basic java server app that has 100 worker threads that do simple HEAD requests on urls. I'm using HttpClient 4.x for this.
A few minutes into the run my program just freezes for a couple minutes and I cannot figure out why. Check out the screen shot of what visual vm monitor reports. You can see it flatline. During this time I'm unable to get a good thread dump and visual vm just freezes until it's unblocked. Does anyone have any ideas on what I can do to try and start debugging this guy?
Visual VM: http://tinypic.com/view.php?pic=2i915bs&s=7
Here is the output when I tried to take a jstack dump while it was frozen:
jstack -F 4325
Attaching to process ID 4325, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 16.3-b01
Deadlock Detection:
No deadlocks found.
Thread 4557: (state = BLOCKED)
Error occurred during stack walking:
sun.jvm.hotspot.debugger.DebuggerException: sun.jvm.hotspot.debugger.DebuggerException: get_thread_regs failed for a lwp
at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$LinuxDebuggerLocalWorkerThread.execute(LinuxDebuggerLocal.java:152)
at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.getThreadIntegerRegisterSet(LinuxDebuggerLocal.java:466)
at sun.jvm.hotspot.debugger.linux.LinuxThread.getContext(LinuxThread.java:65)
at sun.jvm.hotspot.runtime.linux_amd64.LinuxAMD64JavaThreadPDAccess.getCurrentFrameGuess(LinuxAMD64JavaThreadPDAccess.java:92)
at sun.jvm.hotspot.runtime.JavaThread.getCurrentFrameGuess(JavaThread.java:256)
at sun.jvm.hotspot.runtime.JavaThread.getLastJavaVFrameDbg(JavaThread.java:218)
at sun.jvm.hotspot.tools.StackTrace.run(StackTrace.java:76)
at sun.jvm.hotspot.tools.StackTrace.run(StackTrace.java:45)
at sun.jvm.hotspot.tools.JStack.run(JStack.java:60)
at sun.jvm.hotspot.tools.Tool.start(Tool.java:221)
at sun.jvm.hotspot.tools.JStack.main(JStack.java:86)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at sun.tools.jstack.JStack.runJStackTool(JStack.java:118)
at sun.tools.jstack.JStack.main(JStack.java:84)
Caused by: sun.jvm.hotspot.debugger.DebuggerException: get_thread_regs failed for a lwp
at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.getThreadIntegerRegisterSet0(Native Method)
at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.access$800(LinuxDebuggerLocal.java:51)
at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$1GetThreadIntegerRegisterSetTask.doit(LinuxDebuggerLocal.java:460)
at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$LinuxDebuggerLocalWorkerThread.run(LinuxDebuggerLocal.java:127)
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Generating thread dumps on windowsClick the console window and press <CTRL>+BREAK (or SHIFT+CTRL+PAUSE on some keyboards). The thread dump will print directly to the console.
Thread dump was generated due to the long running requester. This was identified on the performance testing. We would like to check the configurations for threshold for the long running requester and thread dump generation threshold, will increase the value if required.
To find the long running threads, highlight all the thread dumps you want to check, and then click on the binoculars: In the pop up dialogue, click start detection, and you'll get your long running threads just below the dumps themselves: In my example, each thread dump has 157 threads.
A deadlock is when two or more threads have a lock over some resources and are both waiting for each other to give up their lock, which causes them to hang. Deadlocks aren't always easy to spot, but they can be extremely problematic when they occur.
I've seen several bug reports about jstack on Linux with a similar trace:
Do you get the same result with a kill -3 <pid>?
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