23.2. Scheduling and Accuracy of Timers

Depending on system load and the scheduling of processes, a process may not be scheduled to run until some short time (i.e., usually some small fraction of a second) after actual expiration of the timer. Notwithstanding this, the expiration of a periodic timer established by setitimer(), or the other interfaces described later in this chapter, will remain regular. For example, if a real-time timer is set to expire every 2 seconds, then the delivery of individual timer events may be subject to the delays just described, but the scheduling of subsequent expirations will nevertheless be at exactly the next 2-second interval. In other words, interval timers are not subject to creeping errors.

Although the timeval structure used by setitimer() allows for microsecond precision, the accuracy of a timer has traditionally been limited by the frequency of the software clock (Section 10.6). If a timer value does not exactly match a multiple of the granularity of the software clock, then the timer value is rounded up. This means that if, for example, we specified an interval timer to go off each 19,100 microseconds (i.e., just over 19 milliseconds), then, assuming a jiffy value of 4 milliseconds, we would actually get a timer that expired every 20 milliseconds.