title: Optimal Number of threads to use
body: <p>Ok so, I'm solving an very paraell problem.
   - generating primes (it's not quiet embarrassingly parallel, since they are written (and read from for checking if they are a factor) from a common source.
  for interest: <a href="http://pastebin.com/sQQLpMgB" rel="nofollow">http://pastebin.com/sQQLpMgB</a></p>
  
  <p>In any case, the thing that inspired me to write this (in part) was realisation of my access to <a href="http://wiki.ucc.asn.au/Motsugo" rel="nofollow">this</a>
  dual Xeon E5520 CPUs (with IIRC 16GB ram to go with it)</p>
  
  <p>So I know that each CPU supports 8 active threads.
  But then there are background processes (and likely other users) using up some of those (in fact probably more that all of those).
  So what is a good rule of thumb as to how many threads make things go faster, before they are being held back by their over head. (I guess  this rule would need to take into acount how many threads can be active at once)</p>