CPU SCHEDULING Basic Concepts

5.1 Basic Concepts

  • Almost all programs have some alternating cycle of CPU number crunching and waiting for I/O of some kind. ( Even a simple fetch from memory takes a long time relative to CPU speeds. )
  • In a simple system running a single process, the time spent waiting for I/O is wasted, and those CPU cycles are lost forever.
  • 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.
  • 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.

5.1.1 CPU-I/O Burst Cycle

  • Almost all processes alternate between two states in a continuing cycle, as shown in Figure 5.1 below :
    • A CPU burst of performing calculations, and
    • An I/O burst, waiting for data transfer in or out of the system.
  • 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 5.2:

5.1.2 CPU Scheduler

  • Whenever the CPU becomes idle, it is the job of the CPU Scheduler ( a.k.a. the short-term scheduler ) to select another process from the ready queue to run next.
  • The storage structure for the ready queue and the algorithm used to select the next process are not necessarily a FIFO queue. There are several alternatives to choose from, as well as numerous adjustable parameters for each algorithm, which is the basic subject of this entire chapter.

5.1.3. Preemptive Scheduling

  • CPU scheduling decisions take place under one of four conditions:
    1. When a process switches from the running state to the waiting state, such as for an I/O request or invocation of the wait( ) system call.
    2. When a process switches from the running state to the ready state, for example in response to an interrupt.
    3. When a process switches from the waiting state to the ready state, say at completion of I/O or a return from wait( ).
    4. When a process terminates.
  • For conditions 1 and 4 there is no choice - A new process must be selected.
  • For conditions 2 and 3 there is a choice - To either continue running the current process, or select a different one.
  • If scheduling takes place only under conditions 1 and 4, the system is said to be non-preemptive, or cooperative. Under these conditions, once a process starts running it keeps running, until it either voluntarily blocks or until it finishes. Otherwise the system is said to be preemptive.
  • Windows used non-preemptive scheduling up to Windows 3.x, and started using pre-emptive scheduling with Win95. Macs used non-preemptive prior to OSX, and pre-emptive since then. Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt.
  • Note that pre-emptive scheduling can cause problems when two processes share data, because one process may get interrupted in the middle of updating shared data structures. Chapter 6 will examine this issue in greater detail.
  • Preemption can also be a problem if the kernel is busy implementing a system call ( e.g. updating critical kernel data structures ) when the preemption occurs. Most modern UNIXes deal with this problem by making the process wait until the system call has either completed or blocked before allowing the preemption Unfortunately this solution is problematic for real-time systems, as real-time response can no longer be guaranteed.
  • Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section. Needless to say, this should only be done in rare situations, and only on very short pieces of code that will finish quickly, ( usually just a few machine instructions. )

5.1.4 Dispatcher

  • The dispatcher is the module that gives control of the CPU to the process selected by the scheduler. This function involves:
    • Switching context.
    • Switching to user mode.
    • Jumping to the proper location in the newly loaded program.
  • The dispatcher needs to be as fast as possible, as it is run on every context switch. The time consumed by the dispatcher is known as dispatch latency.
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