title: Unable to get thread dump? Any ideas why my app blocks?
body: <p>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. </p>
  
  <p>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?</p>
  
  <p>Visual VM: <a href="http://tinypic.com/view.php?pic=2i915bs&amp;s=7" rel="noreferrer">http://tinypic.com/view.php?pic=2i915bs&amp;s=7</a></p>
  
  <p>Here is the output when I tried to take a jstack dump while it was frozen:</p>
  
  <pre><code>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)
  </code></pre>