title: cpu efficiency formula
body: <p>I have a question like this :</p>
  
  <p>Measurement of a certain system have shown that the average process runs for a time T before blocking on IO. A process switch requires a time S, which is effectively wasted ( overhead ). For round robin scheduling with quantum Q, give a formula for the CPU efficiency for each of the following</p>
  
  <pre><code>( a ) Q = INFINITY
  ( b ) Q &gt; T
  ( c ) S &lt; Q &lt; T
  ( d ) Q = S
  ( e ) Q -&gt; 0
  </code></pre>
  
  <p>I know how to do the a,b,d and e, but for c, the answer is T/(T + S * T/Q) = Q/(Q + S). It means the total times context switching occurs is T/Q which makes me confusing, let say T = 3, Q = 2, the process run for 2 units and switch to another process, then later it is switched back to execute and finish, then switch to another process again, so it is 2 switches which is = roof(T/Q); but based on the answer, there is only 1 switching, so there is no different between running in 1 round and 2 rounds? Could anyone explain it to me and what exactly CPU efficiency is.</p>