CPU-I/O Burst Cycle

• The success of CPU scheduling depends on an observed property of processes:
  • Process execution consists of a cycle of CPU execution and I/O wait. Processes alternate between these two states.
  • Process execution begins with a CPU burst. That is followed by an I/O burst, which is followed by another CPU burst, then another I/O burst, and so on.
• Eventually, the final CPU burst ends with a system request to terminate execution (see Fig. 10).

  Figure 10: Alternating sequence of CPU and I/O bursts.

  
• The durations of CPU bursts have been measured extensively. They tend to have a frequency curve similar to that shown in Fig. 11.

  Figure 11: Histogram of CPU-burst durations.

  
• The curve is generally characterized as exponential or hyperexponential, with a large number of short CPU bursts and a small number of long CPU bursts.
  • An I/O-bound program typically has many short CPU bursts.
  • A CPU-bound program might have a few long CPU bursts.
• This distribution can be important in the selection of an appropriate CPU-scheduling algorithm.
• Nearly all processes alternate bursts of computing with (disk) I/O requests, as shown in Fig. 12.

  Figure 12: Bursts of CPU usage alternate with periods of waiting for I/O. (a) A CPU-bound process. (b) An I/O-bound process.

  
• Some processes, such as the one in Fig. 12a, spend most of their time computing (CPU-bound), while others, such as the one in Fig. 12b, spend most of their time waiting for I/O (I/O-bound).
• Having some CPU-bound processes and some I/O-bound processes in memory together is a better idea than first loading and running all the CPU-bound jobs and then when they are finished loading and running all the I/O-bound jobs (a careful mix of processes).