Chapter 6: CPU Scheduling

• • •

Basic Concepts Scheduling Criteria Scheduling Algorithms

Operating System Concepts

6.1

Basic Concepts • Maximum CPU utilization obtained with multiprogramming.

• CPU–I/O Burst Cycle
– Process execution consists of a cycle of CPU execution and I/O wait. – Example: Alternating Sequence of CPU And I/O Bursts – In an I/O – bound program would have many very short CPU bursts. – In a CPU – bound program would have a few very long CPU bursts.
Operating System Concepts 6.2

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CPU Scheduler • The CPU scheduler (short-term scheduler) selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.

• A ready queue may be implemented as a FIFO queue, priority queue, a tree, or an unordered linked list.

• CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state (ex., I/O request). 2. Switches from running to ready state (ex., Interrupts occur). 3. Switches from waiting to ready state (ex., Completion of I/O). 4. Terminates.

• Scheduling under 1 and 4 is nonpreemptive; otherwise is called 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.
Operating System Concepts 6.3

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.

Operating System Concepts

6.4

2

Scheduling Criteria

• CPU utilization – keep the CPU as busy as possible. • Throughput – number of processes that complete their execution per time unit.

• Turnaround time – amount of time to execute a particular process; which is the interval from time of submission of a process to the time of completion (includes the sum of periods spent waiting to get into memory, waiting in ready queue, executing on the CPU, and doing I/O).

• Waiting time – sum of the periods spent waiting in the ready queue. • Response time – the time from the submission of a request until the first response is produced (i.e., the amount of time it takes to start responding, but not the time that it takes to output that response).
Operating System Concepts 6.5

Optimization Criteria

• Max CPU utilization • Max throughput • Min turnaround time • Min waiting time • Min response time

Operating System Concepts

6.6

3

Scheduling Algorithms

• CPU scheduling deals with the problem of choosing a process from the ready queue to be executed by the CPU.

• The following CPU scheduling algorithms will be described: – First-Come, First-Served (FCFS). – Shortest-Job-First (SJF). – Priority. – Round-Robin (RR). – Multilevel Queue. – Multilevel Feedback Queue.
Operating System Concepts 6.7

First-Come, First-Served (FCFS) Scheduling

• It is the simplest CPU scheduling algorithm. • The process that requests the CPU first is allocated the CPU first.

• The average waiting time under FCFS is long.
Completion 3 2 1 CPU

Ready Queue

Operating System Concepts

6.8

4

First-Come, First-Served (FCFS) Scheduling

• Example:

Process P1 P2 P3

Burst Time (milliseconds) 24 3 3

• Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
P1 0 24 P2 27 P3 30

• Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27) / 3 = 17 (milliseconds)
Operating System Concepts 6.9

FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order P2 , P3 , P1 .

• The Gantt chart for the schedule is:
P2 0 3 P3 6 P1 30

• Waiting time for P1 = 6; P2 = 0; P3 = 3 • Average waiting time: (6 + 0 + 3) / 3 = 3 (milliseconds) • Much better than previous case. • The order of processes in FCFS queue is important.
Operating System Concepts 6.10

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FCFS Scheduling (Cont.)

• Another example, assume you have one CPU-bound process and many I/O-bound processes. – CPU executes the CPU-bound process and I/Obound processes in ready queue after they used I/O devices (waiting). – So, I/O devices are idle. – All I/O-bound processes have very short CPU bursts.

• The FCFS algorithm is nonpreemptive (i.e., the CPU keeps the process until either terminating or requesting I/O).

Operating System Concepts

6.11

Shortest-Job-First (SJF) Scheduling

• Associate with each process the length of its next CPU burst.
Use these lengths to schedule the process with the shortest time.

• Two schemes:
– Nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst. – Preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is known as the Shortest-Remaining-Time-First (SRTF).

• If two processes have the same length next CPU burst, FCFS scheduling is used.

• SJF is optimal – gives minimum average waiting time for a given set of processes.
Operating System Concepts 6.12

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Example of Non-Preemptive SJF

• Example: Process
P1 P2 P3 P4

Arrival Time 0.0 2.0 4.0 5.0

Burst Time 7 4 1 4

• SJF (non-preemptive)
P1 0 3 7 P3 8 P2 12 P4 16

• Average waiting time = (0 + 6 + 3 + 7) / 4 = 4
Operating System Concepts 6.13

Example of Non-Preemptive SJF (Cont.)

• If we used FCFS, then the average waiting time would be: (0 + 5 + 7 + 7) / 4 = 4.75

• The SJF gives less average waiting time than FCFS. • The SJF is a nonpreemptive in which the waiting process with the smallest estimated run-time-tocompletion is run next. Therefore, SJF is not useful in timesharing environments.

• Both FCFS and SJF are not useful for timesharing environments, because they are nonpreemptive.

Operating System Concepts

6.14

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Example of Preemptive SJF

• Example:

Process P1 P2 P3 P4

Arrival Time 0.0 2.0 4.0 5.0

Burst Time 7 4 1 4

• SJF (preemptive) [Shortest-Remaining-Time-First (SRTF)]
P1 0 2 P2 4 P3 5 P2 7 P4 11 P1 16

• Average waiting time = (9 + 1 + 0 +2) / 4 = 3
Operating System Concepts 6.15

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).

• Equal priority processes are scheduled in FCFS. • SJF is a special case of general priority scheduling, where priority is the predicted next CPU burst time.

• Priorities can be defined either internally or externally. • Internally use of some measurable quantity or quantities to compute the priority of a process. For example, memory requirements, number of open files, ratio of average I/O burst to average CPU burst have been used in computing priorities.

• Externally priority is set by criteria that is external to the O.S.; such as importance of the process.

Operating System Concepts

6.16

8

Priority Scheduling (Cont.)

• Priority can be either preemptive or nonpreemptive. • A preemptive priority will preempt the CPU if the newly arrived process is higher than the priority of the currently running process.

• A nonpreemptive priority will simply put the new highest priority process at the head of the ready queue.

• Problem ≡ Starvation – low priority processes may never execute. • Solution ≡ Aging – as time progresses increase the priority of the process, so eventually the process will become the highest priority and will gain the CPU (i.e., the more time is spending a process in ready queue waiting, its priority becomes higher and higher).
Operating System Concepts 6.17

Example of Nonpreemptive Priority
Process P1 P2 P3 P4 P5 Burst Time 10 1 2 1 5 Priority 3 1 3 4 2

• Gantt Chart:
P2 0 1 P5 6 P1 16 P3 18 P4 19

• Average waiting time = (0 + 1 + 6 + 16 + 18) / 5 = 8.2
Operating System Concepts 6.18

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Round Robin (RR) • The Round Robin is designed for time sharing systems. • The RR is similar to FCFS, but preemption is added to switch between processes.

• Each process gets a small unit of CPU time (time quantum or time slice ), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. That is, after the time slice is expired an interrupt will occur and a context switch.
Ready Queue Completion 3 2 1 CPU

Preemption

Time Slice

Operating System Concepts

6.19

Round Robin (Cont.)

• 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.

• The performance of RR depends on the size of the time slice q : – If q very large (infinite) ⇒ FIFO – If q very small ⇒ RR is called processor sharing, and context switch increases. So, q must be large with respect to context switch, otherwise overhead is too high.

Operating System Concepts

6.20

10

Example: RR with Time Quantum = 20
Process P1 P2 P3 P4 Burst Time 53 17 68 24 Waiting Time of each Process 0+(77-20)+(121-97)=81 20 37+(97-57)+(134-117)=94 57+(117-77)=97

• The Gantt chart is:
P1 0 20 P2 37 P3 57 P4 77 P1 P3 97 117 P4 P1 P3 P3

121 134 154 162

• Average Waiting Time = (81+20+94+97) / 4 = 73
Operating System Concepts 6.21

How a Smaller Time Quantum Increases Context Switches

Operating System Concepts

6.22

11

Multilevel Queue

• Ready queue is partitioned into separate queues:
– foreground (interactive) – background (batch)

• Each queue has its own scheduling algorithm:
– foreground – RR – background – FCFS

• Scheduling must be done between the queues.
– Fixed priority scheduling; i.e., serve all from foreground then from background. Possibility of starvation. – Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; 80% to foreground in RR 20% to background in FCFS
Operating System Concepts 6.23

Multilevel Queue Scheduling

Operating System Concepts

6.24

12

Multilevel Feedback Queue

• A process can move between various queues; aging can be implemented this way.

• If a process waits too long in a lower-priority queue may be moved to a higher-priority queue (this form of aging to prevent starvation).

• If a process uses too much CPU time, it will be moved to lower-priority queues. This leaves I/O bound and interactive processes in the higher-priority queues.

• In general, the multilevel feedback queue scheduling algorithm is the most complex.

Operating System Concepts

6.25

Multilevel Feedback Queues

Operating System Concepts

6.26

13

Example of Multilevel Feedback Queue

• Three queues:
– Q0 – time quantum 8 milliseconds – Q1 – time quantum 16 milliseconds – Q2 – FCFS

• Scheduling:
– A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. – At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.

• The Multilevel Feedback Queue Scheduling is preemptive.
Operating System Concepts 6.27

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