title: Is Concurrent Mark Sweep (CMS) a stop the world event?
body: <p>I see many unloading of classes and my entire system will hang during that period of time..</p>
  
  <pre><code>[Unloading class sun.reflect.GeneratedMethodAccessor117]
  [Unloading class sun.reflect.GeneratedConstructorAccessor1896]
  [Unloading class sun.reflect.GeneratedSerializationConstructorAccessor485]
  [Unloading class sun.reflect.GeneratedSerializationConstructorAccessor579]
  .... // about 1700 of them
  </code></pre>
  
  <p>at the same time I do not see a spike in perm space, so it doesn't seems to be a GC event.</p>
  
  <p>I wish to know the following</p>
  
  <p>IS Concurrent Mark Sweep collection a stop the world event?</p>
  
  <p>Does it happen even when the perm space is not full?</p>