title: Full GC real time is much more that user+sys times
body: <p>We have a Web Java based application running on JBoss with allowed maximum heap size of about 1.2 GB (total machine physical memory is 2 GB). At some point the application stops responding (to clients) for several minutes. After some analysis we found out that the culprit is the Full GC. Here's an excerpt from the verbose GC log:</p>
  
  <p>74477.402: [Full GC [PSYoungGen: 3648K->0K(332160K)] [PSOldGen: 778476K->589497K(819200K)] 782124K->589497K(1151360K) [PSPermGen: 102671K->102671K(171328K)], 646.1546860 secs] [<strong>Times: user=3.84 sys=3.72, real=646.17 secs</strong>] </p>
  
  <p>What I don't understand is how is it possible that the <strong>real time</strong> spent on Full GC is about 11 minutes (646 seconds), while <strong>user+sys times</strong> are just 7.5 seconds. 7.5 seconds sound to me much more logical time to spend for cleaning &lt;200 MB from the old generation. Where does all the other time go?</p>
  
  <p>Thanks a lot.</p>