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Unformatted text preview: al Counting. With a timer interval of 10 ms, every 10 ms segment is assigned to a process as part of either its user (u) or system (s) time. This accounting provides only an approximate measure of program execution time. 456 CHAPTER 9. MEASURING PROGRAM EXECUTION TIME 9.2.1 Operation
The operating system maintains counts of the amount of user time and the amount of system time used by each process. When a timer interrupt occurs, the operating system determines which process was active and increments one of the counts for that process by the timer interval. It increments the system time if the system was executing in kernel mode, and the user time otherwise. The example shown in Figure 9.7(a) indicates this accounting for the two processes. The tick marks indicate the occurrences of timer interrupts. Each is labeled by the count that gets incremented: either Au or As for process A’s user or system time, or Bu or Bs for process B’s user or system time. Each tick mark is labeled according to the activity to its immediate left. The ﬁnal accounting shows that process A used a total of 150 milliseconds: 110 of use...
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