title: User CPU time deviation. What is it caused by?
body: <p>I am wondering why the time command in Unix always outputs different user CPU time. It is said it is time that CPU spends executing user code of the needed process, so it excludes tasks that are managed by the kernel:</p>
  
  <ul>
  <li>Any I/O or other hardware waits and interrupts, also cache management</li>
  <li>Other processes' intervention (taking control away)</li>
  <li>All things that user code does not know about</li>
  </ul>
  
  <p>But for a simple C program with bubble-sorting of 1000000 elements it always shows user CPU time ranging from 0.3 to 1.0 seconds.</p>
  
  <p>I have found little information about that in classic books about kernels and operation systems. Please, enlighten me, somebody.</p>