title: jstat: difference between OGC & OC, PGC & PC
body: <p>I'm learning about jstat and what it can tell me about the JVM's different generations. From <a href="http://docs.oracle.com/javase/6/docs/technotes/tools/share/jstat.html" rel="noreferrer">the jstat docs</a> I understand the new gen is made up of eden, s0 and s1. For example, if you do the math on the following, you see that NGC = EC + S0C + S1C. Great stuff.</p>
  
  <pre><code>$ jstat -gccapacity -t 21830 5000
  Timestamp        NGCMN    NGCMX     NGC     S0C   S1C       EC      OGCMN      OGCMX           OGC         OC      PGCMN    PGCMX     PGC       PC     YGC    FGC 
         248767.4   2624.0  87360.0   6656.0  640.0  640.0   5376.0     5376.0   174784.0    12840.0    12840.0  21248.0 131072.0  34304.0  34304.0    457    73
         248772.4   2624.0  87360.0   6656.0  640.0  640.0   5376.0     5376.0   174784.0    12840.0    12840.0  21248.0 131072.0  34304.0  34304.0    457    73
         248777.3   2624.0  87360.0   6656.0  640.0  640.0   5376.0     5376.0   174784.0    12840.0    12840.0  21248.0 131072.0  34304.0  34304.0    457    73
  </code></pre>
  
  <p>I'm wondering what's the difference between:</p>
  
  <ul>
  <li>OGC (Current old generation capacity (KB)) and </li>
  <li>OC (Current old space capacity (KB)). </li>
  </ul>
  
  <p>And similarly for:</p>
  
  <ul>
  <li>PGC (Current Permanent generation capacity (KB)) and</li>
  <li>PC (Current Permanent space capacity (KB)).</li>
  </ul>
  
  <p>Each pair has the same value, at least for me, right now. Is there ever anything in the old generation beside the old space?</p>
  
  <p><hr>
  Edit: I don't think there is a difference, but I'll leave this question up just in case.</p>