the short-term scheduler ) to select another process from the ready queue to run next. Whenever the CPU becomes idle, it is the job of the CPU Scheduler ( a.k.a.CPU bursts vary from process to process, and from program to program, but an extensive study shows frequency patterns similar to that shown in Figure 6.2:įigure 6.2 - Histogram of CPU-burst durations.An I/O burst, waiting for data transfer in or out of the system.įigure 6.1 - Alternating sequence of CPU and I/O bursts.A CPU burst of performing calculations, and.Almost all processes alternate between two states in a continuing cycle, as shown in Figure 6.1 below :.The challenge is to make the overall system as "efficient" and "fair" as possible, subject to varying and often dynamic conditions, and where "efficient" and "fair" are somewhat subjective terms, often subject to shifting priority policies.A scheduling system allows one process to use the CPU while another is waiting for I/O, thereby making full use of otherwise lost CPU cycles.In a simple system running a single process, the time spent waiting for I/O is wasted, and those CPU cycles are lost forever.( Even a simple fetch from memory takes a long time relative to CPU speeds. ) Almost all programs have some alternating cycle of CPU number crunching and waiting for I/O of some kind.Abraham Silberschatz, Greg Gagne, and Peter Baer Galvin, "Operating System Concepts, Ninth Edition ", Chapter 6.
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