Chapter 5: CPU Scheduling

CPU–I/O Burst Cycle

Basic Concepts

• Maximum CPU utilization obtained with multiprogramming

• — Process execution consists of a cycle of CPU execution and I/O wait

I/O burst

Basic Concepts

• CPU burst followed by —

• CPU burst distribution is of main concern

Histogram of CPU-burst Times

—

• Large number of short bursts

• Small number of longer bursts

CPU scheduler

selects from among the processes in ready queue, and allocates a CPU core to one of them

• Queue may be ordered in various ways

1. Switches from running to waiting state

2. Switches from running to ready state

3. Switches from waiting to ready

4. Terminates

CPU scheduling decisions may take place when a process (4)

• 1 and 4

• 2 and 3

• For situations , there is no choice in terms of scheduling. A new process (if one exists in the ready queue) must be selected for execution.

• For situations —, however, there is a choice.

• nonpreemptive

• preemptive

• When scheduling takes place only under circumstances 1 and 4, the scheduling scheme is —

• Otherwise, it is—

Nonpreemptive scheduling

Under —, once the CPU has been allocated to a process, the process keeps the CPU until it releases it either by terminating or by switching to the waiting state.

preemptive scheduling algorithms

Virtually all modern operating systems including Windows, MacOS, Linux, and UNIX use

race conditions

Preemptive scheduling can result in — when data are shared among several processes.

• Consider the case of two processes that share data. While one process is updating the data, it is preempted so that the second process can run. The second process then tries to read the data, which are in an inconsistent state.

Dispatcher module

— gives control of the CPU to the process selected by the CPU 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

• CPU utilization

• Throughput

• Turnaround time

• Waiting time

• Response time

5 Scheduling Criteria

CPU utilization

keep the CPU as busy as possible

Throughput

# of processes that complete their execution per time unit

Turnaround time

amount of time to execute a particular process

Waiting time

amount of time a process has been waiting in the ready queue

Response time

amount of time it takes from when a request was submitted until the first response is produced.

• Max CPU utilization

• Max throughput

• Min turnaround time

• Min waiting time

• Min response time

5 Scheduling Algorithm Optimization Criteria

First- Come, First-Served (FCFS) Scheduling

(0 + 24 + 27)/3 = 17

—

Waiting time for P1 = 0; P2 = 24; P3 = 27

Average waiting time: —

(6 + 0 + 3)/3 = 3

Waiting time for P1 = 6; P2 = 0 ; P3 = 3

Average waiting time: —

Much better than previous case

Convoy effect

— short process behind long process

• Consider one CPU-bound and many I/O-bound processes

Shortest-Job-First (SJF) Scheduling

• optimal

• Associate with each process the length of its next CPU burst

  • Use these lengths to schedule the process with the shortest time

• — gives minimum average waiting time for a given set of processes
  

shortest-remaining-time-first

• Preemptive version called —

• How do we determine the length of the next CPU burst?

  • Could ask the user

  • Estimate

• (9+24+16+3) / 4 =13

• (3 + 16 + 9 + 0) / 4 = 7

• Average turnaround time(finish - arrival) =

• Average waiting time(turnaround - burst) =

shortest predicted next CPU burst

Determining Length of Next CPU Burst

• Can only estimate the length which should be similar to the previous one

  • Then pick process with —

• exponential averaging

• ½

Determining Length of Next CPU Burst

• Can be done by using the length of previous CPU bursts, using —

• Commonly, α set to —

less weight

Examples of Exponential Averaging

Since both α and (1 - α) are less than or equal to 1, each successor predecessor term has — than its predecessor

Shortest Remaining Time First Scheduling

—

• Preemptive version of SJN

• Whenever a new process arrives in the ready queue, the decision on which process to schedule next is redone using the SJN algorithm.

• Is SRT more “optimal” than SJN in terms of the minimum average waiting time for a given set of processes?

[(10-1)+(1-1)+(17-2)+(5-3)]/4 = 26/4 = 6.5

Example of Shortest-remaining-time-first

Average waiting time =

Round Robin (RR)

• Each process gets a small unit of CPU time (time quantum q), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.

• If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.

• FIFO (FCFS)

• RR

Round Robin (RR)

• Timer interrupts every quantum to schedule next process

• Performance

  • q large ⇒ —

  • q small ⇒ —

• Note that q must be large with respect to context switch, otherwise overhead is too high

• response

• context switch time

Example of RR with Time Quantum = 4

• Typically, higher average turnaround than SJF, but better —

• q should be large compared to —

  • q usually 10 milliseconds to 100 milliseconds,

  • Context switch < 10 microseconds

Time Quantum and Context Switch Time

process time, quantum, context switches

Turnaround Time Varies With The Time Quantum

80% of CPU bursts should be shorter than q

Priority Scheduling

• A priority number (integer) is associated with each process ▪

• The CPU is allocated to the process with the highest priority (smallest integer ≡ highest priority)

  • Preemptive

  • Nonpreemptive

SJF

is priority scheduling where priority is the inverse of predicted next CPU burst time

• Starvation

• Aging

• Problem ≡ — low priority processes may never execute

• Solution ≡ — as time progresses increase the priority of the process

8.2

Example of Priority Scheduling

Average waiting time =

Priority Scheduling w/ Round-Robin

Run the process with the highest priority. Processes with the same priority run round-robin

Multilevel Queue

The ready queue consists of multiple queues

• Number

• Scheduling algorithms

• needs service

• Scheduling

Multilevel queue scheduler defined by the following parameters (4)

• — of queues

• — for each queue

• Method used to determine which queue a process will enter when that process —

• — among the queues