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Nội dung Text: Bài giảng Hệ điều hành nâng cao - Chapter 5: CPU Scheduling
- Chapter 5: CPU Scheduling
Operating System Concepts – 8th Edition
Operating System Concepts – 8th Edition 5.1 Silberschatz, Galvin and Gagne ©2009
- Chapter 5: CPU Scheduling
s Basic Concepts
s Scheduling Criteria
s Scheduling Algorithms
s Thread Scheduling
s Multiple-Processor Scheduling
s Operating Systems Examples
s Algorithm Evaluation
Operating System Concepts – 8th Edition 5.2 Silberschatz, Galvin and Gagne ©2009
- Objectives
s To introduce CPU scheduling, which is the basis for multiprogrammed operating systems
s To describe various CPU-scheduling algorithms
s To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system
Operating System Concepts – 8th Edition 5.3 Silberschatz, Galvin and Gagne ©2009
- Basic Concepts
s Maximum CPU utilization obtained with multiprogramming
s CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait
s CPU burst distribution
Operating System Concepts – 8th Edition 5.4 Silberschatz, Galvin and Gagne ©2009
- Alternating Sequence of CPU and
I/O Bursts
Operating System Concepts – 8th Edition 5.5 Silberschatz, Galvin and Gagne ©2009
- Histogram of CPU-burst Times
Operating System Concepts – 8th Edition 5.6 Silberschatz, Galvin and Gagne ©2009
- CPU Scheduler
s Selects from among the processes in ready queue, and allocates the CPU to one of them
q Queue may be ordered in various ways
s 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
s Scheduling under 1 and 4 is nonpreemptive
s All other scheduling is preemptive
q Consider access to shared data
q Consider preemption while in kernel mode
q Consider interrupts occurring during crucial OS activities
Operating System Concepts – 8th Edition 5.7 Silberschatz, Galvin and Gagne ©2009
- Dispatcher
s Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this
involves:
q switching context
q switching to user mode
q jumping to the proper location in the user program to restart that program
s Dispatch latency – time it takes for the dispatcher to stop one process and start another running
Operating System Concepts – 8th Edition 5.8 Silberschatz, Galvin and Gagne ©2009
- Scheduling Criteria
s CPU utilization – keep the CPU as busy as possible
s Throughput – # of processes that complete their execution per time unit
s Turnaround time – amount of time to execute a particular process
s Waiting time – amount of time a process has been waiting in the ready queue
s 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)
Operating System Concepts – 8th Edition 5.9 Silberschatz, Galvin and Gagne ©2009
- Scheduling Algorithm Optimization Criteria
s Max CPU utilization
s Max throughput
s Min turnaround time
s Min waiting time
s Min response time
Operating System Concepts – 8th Edition 5.10 Silberschatz, Galvin and Gagne ©2009
- First-Come, First-Served (FCFS) Scheduling
Process Burst Time
P1 24
P2 3
P3 3
s Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
P1 P2 P3
0 24 27 30
s Waiting time for P1 = 0; P2 = 24; P3 = 27
s Average waiting time: (0 + 24 + 27)/3 = 17
Operating System Concepts – 8th Edition 5.11 Silberschatz, Galvin and Gagne ©2009
- FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order:
P2 , P3 , P1
s The Gantt chart for the schedule is:
P2 P3 P1
0 3 6 30
s Waiting time for P1 = 6; P2 = 0; P3 = 3
s Average waiting time: (6 + 0 + 3)/3 = 3
s Much better than previous case
s Convoy effect - short process behind long process
q Consider one CPU-bound and many I/O-bound processes
Operating System Concepts – 8th Edition 5.12 Silberschatz, Galvin and Gagne ©2009
- Shortest-Job-First (SJF) Scheduling
s Associate with each process the length of its next CPU burst
q Use these lengths to schedule the process with the shortest time
s SJF is optimal – gives minimum average waiting time for a given set of processes
q The difficulty is knowing the length of the next CPU request
q Could ask the user
Operating System Concepts – 8th Edition 5.13 Silberschatz, Galvin and Gagne ©2009
- Example of SJF
ProcessArriva l Time Burst Time
P1 0.0 6
P2 2.0 8
P3 4.0 7
P4 5.0 3
s SJF scheduling chart
P4 P1 P3 P2
0 3 9 16 24
s Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
Operating System Concepts – 8th Edition 5.14 Silberschatz, Galvin and Gagne ©2009
- Determining Length of Next CPU Burst
s Can only estimate the length – should be similar to the previous one
q Then pick process with shortest predicted next CPU burst
s Can be done by using the length of previous CPU bursts, using exponential averaging
1. t n = actual length of n th CPU burst
2. τ n +1 = predicted value for the next CPU burst
3. α, 0 ≤ α ≤ 1
4. Define :
s Commonly, α set to ½
s Preemptive version called shortest-remaining-time-first
τ n =1 = α t n + (1 − α )τ n .
Operating System Concepts – 8th Edition 5.15 Silberschatz, Galvin and Gagne ©2009
- Prediction of the Length of the
Next CPU Burst
Operating System Concepts – 8th Edition 5.16 Silberschatz, Galvin and Gagne ©2009
- Examples of Exponential Averaging
s =0
q n+1 = n
q Recent history does not count
s =1
q n+1 = tn
q Only the actual last CPU burst counts
s If we expand the formula, we get:
n+1 = tn+(1 - ) tn -1 + …
+(1 - )j tn -j + …
+(1 - )n +1 0
s Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its
predecessor
Operating System Concepts – 8th Edition 5.17 Silberschatz, Galvin and Gagne ©2009
- Example of Shortest-remaining-time-first
s Now we add the concepts of varying arrival times and preemption to the analysis
ProcessA arri Arrival TimeTBurst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
s Preemptive SJF Gantt Chart
P1 P2 P4 P1 P3
0 1 5 10 17 26
s Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5 msec
Operating System Concepts – 8th Edition 5.18 Silberschatz, Galvin and Gagne ©2009
- Priority Scheduling
s A priority number (integer) is associated with each process
s The CPU is allocated to the process with the highest priority (smallest integer highest priority)
q Preemptive
q Nonpreemptive
s SJF is priority scheduling where priority is the inverse of predicted next CPU burst time
s Problem Starvation – low priority processes may never execute
s Solution Aging – as time progresses increase the priority of the process
Operating System Concepts – 8th Edition 5.19 Silberschatz, Galvin and Gagne ©2009
- Example of Priority Scheduling
ProcessA arri Burst TimeT Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
s Priority scheduling Gantt Chart
P2 P5 P1 P3 P4
0 1 6 16 18 19
s Average waiting time = 8.2 msec
Operating System Concepts – 8th Edition 5.20 Silberschatz, Galvin and Gagne ©2009
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