CPU scheduling is the basis of multi-programmed operating systems. By switching the CPU among processes, the operating system can make the computer more productive
- To introduce CPU scheduling, which is the basis for multi-programmed operating systems
- To describe various CPU-scheduling algorithms
- To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system
- To examine the scheduling algorithms of several operating systems
3. Objectives
CPU scheduling is the basis of multiprogrammed operating systems. By
switching the CPU among processes, the operating system can make the
computer more productive
• To introduce CPU scheduling, which is the basis for multiprogrammed
operating systems
• To describe various CPU-scheduling algorithms
• To discuss evaluation criteria for selecting a CPU-scheduling algorithm for
a particular system
• To examine the scheduling algorithms of several operating systems
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4. Basic Concepts
• In a single-processor system, only one process can run at a time
• Others must wait until the CPU is free and can be rescheduled
• The objective of multiprogramming is to have some process running at
all times, to maximize CPU utilization
• A process is executed until it must wait, typically for the completion of
some I/O request
• In a simple computer system, the CPU then just sits idle. All this waiting
time is wasted; no useful work is accomplished
• With multiprogramming, we try to use this time productively, maximum
CPU utilization
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5. Basic Concepts
• With multiprogramming, we try to use this time productively
• Several processes are kept in memory at one time
• When one process has to wait, the operating system takes the CPU
away from that process and gives the CPU to another process
• This pattern continues...
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6. Basic Concepts
• Process execution consists of a
cycle of CPU execution and I/O
wait - CPU - I/O Burst Cycle
• CPU Burst - when the process is
being executed in the CPU
• I/O Burst - when the CPU is
waiting for I/O for further exection
• CPU burst followed by I/O burst
• Eventually, the final CPU burst
ends with a system request to
terminate execution
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7. Histogram of CPU-burst Times
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• An I/O-bound program typically has many short CPU bursts
• A CPU-bound program might have a few long CPU bursts.
8. CPU Scheduler
• Short-term/CPU scheduler selects from among the processes in ready
queue, and allocates the CPU to one of them
• Queue may be ordered in various ways
• CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4. Terminates
• For situations 1 and 4, there is no choice in terms of scheduling
A new process (if one exists in the ready queue) must be selected for
execution
• However, there is a choice for situations 2 and 3
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9. CPU Scheduler
• When scheduling takes place only under circumstances 1 and 4, we say
that the scheduling scheme is nonpreemptive or cooperative
• Otherwise, it is preemptive
• Under nonpreemptive scheduling, once the CPU has been allocated to
a process, the process keeps the CPU until it releases the CPU either by
terminating or by switching to the waiting state
• Under preemptive scheduling, CPU has been taken away from a
process, before the process has completed its execution or before it is
swicting to the waiting state
• Scheduling under 1 and 4 is nonpreemptive
• All other scheduling is preemptive
• Consider access to shared data
• Consider preemption while in kernel mode
• Consider interrupts occurring during crucial OS activities
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10. Dispatcher
• Dispatcher module gives control of the CPU to the process selected by
the short-term scheduler; this involves:
• switching context
• switching to user mode
• jumping to the proper location in the user program to restart that
program
• Dispatch latency – time it takes for the dispatcher to stop one process
and start another running
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11. Scheduling Criteria
• CPU utilization –
• keep the CPU as busy as possible
• Conceptually, CPU utilization can range from 0 to 100 percent
• In a real system, it should range from 40 percent (for a lightly loaded
system) to 90 percent (for a heavily loaded system).
• Throughput –
• Number of processes that complete their execution per time unit
• For long processes, this rate may be one process per hour
• For short transactions, it may be ten processes per second
• Turnaround time –
• amount of time to execute a particular process
• The interval from the time of submission of a process to the time of
completion is the turnaround time
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12. Scheduling Criteria
• Turnaround time –
• Turnaround time is the sum of the periods spent waiting to get into
memory, waiting in the ready queue, executing on the CPU, and doing
I/O
• Waiting time –
• The CPU-scheduling algorithm does not affect the amount of time
during which a process executes or does I/O
• It affects only the amount of time that a process spends waiting in the
ready queue
• Waiting time is the sum of the periods spent waiting in the ready
queue
• Response time –
• amount of time it takes from when a request was submitted until the
first response is produced, not output (for time-sharing environment)
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13. Scheduling Algorithm Optimization Criteria
• Max CPU utilization
• Max throughput
• Min turnaround time
• Min waiting time
• Min response time
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