title: Using XCode and instruments to improve iPhone app performance
body: <p>I've been experimenting with Instruments off and on for a while and and I still can't do the following (with any sensible results):  determine or estimate the average runtime of a function that's called many times.</p>
  
  <p>For example if I'm driving my gameLoop at 60 Hz with a CADisplayLink I'd like to see how long the loop takes to run on average... 10 ms? 30 ms etc.  </p>
  
  <p>I've come close with the "CPU activity" instrument but the results are inconsistent or don't make sense.  The time profiler seems promising but all I can get is "% of runtime"... and I'd like an <em>actual</em> runtime.</p>