title: What is the exact definition of 'process preemption'?
body: <p>Wikipedia says:</p>
  <blockquote>
  <p>In computing, preemption is the act of temporarily interrupting a task being carried out by a computer system, without requiring its cooperation, and with the intention of resuming the task at a later time.</p>
  </blockquote>
  <p>Other sources say:</p>
  <blockquote>
  <p>[...] preemption means forcefully taking away of the processor from one process and allocating it to another process. [<em>Operating Systems (Self Edition 1.1), Sibsankar Haldar</em>]</p>
  <p>Preemption of a program occurs when an interrupt arises during its execution and the scheduler selects some other programs for execution. [<em>Operating Systems: a Concept-based Approach, 2E, D. M. Dhamdhere</em>]</p>
  </blockquote>
  <p>So, what I understood is that we have process preemption if the process is interrupted (by a hardware interrupt, i.e. I/O interrupt or timer interrupt) <strong>and</strong> the scheduler, invoked after handling the interrupt, selects another process to run (according to the CPU scheduling algorithm). If the scheduler selects the interrupted process we have no process preemption (<strong>interrupts do not necessarily cause preemption</strong>).</p>
  <p>But I found many other sources that define preemption in the following way:</p>
  <blockquote>
  <p>Preemption is the forced deallocation of the CPU from a program. [<em>Operating Systems: a Concept-based Approach, 2E, D. M. Dhamdhere</em>]</p>
  </blockquote>
  <p>You can see that the same book reports two different definitions of preemption. In the latter it is not mentioned that the CPU must be allocated to another process. According to this definition, preemption is just another name for 'interruption'. When a hardware interrupt arises, the process is interrupted (it switches from &quot;Running&quot; to &quot;Ready&quot; state) or preempted.</p>
  <p>So my question is: which of the two definitions is correct? I'm quite confused.</p>