Transcript cpu scheduling

CH 5. CPU Scheduling
2015-11-02
5.1 Basic Concepts

CPU Scheduling
 context switching


CPU switching for another process
saving old PCB and loading new PCB
 dispatcher


give control of the CPU to the process selected by the CPU
scheduler
CPU-I/O Burst Cycle
 Process execution begins with a CPU burst
5.1 Basic Concepts
Load store
add store
read from file
Wait for I/O
CPU burst
160
I/O burst
140
Wait for I/O
CPU burst
I/O burst
frequency
Store increment
index
write to file
120
100
80
60
40
Load store
add store
read from file
20
CPU burst
0
8
16
24
32
40
burst duration (milliseconds)
Wait for I/O
I/O burst
Alternating sequence of CPU and I/O bursts
Histogram of CPU-burst times
5.2 Scheduling Criteria

Performance comparison criteria of CPU scheduling
algorithm





CPU utilization : 40% ~ 90%
throughput : finished job # per time unit
turnaround time : waiting time in the Ready Queue
response time : interactive system
preemptive & nonpreemptive Scheduling
 context switching : pure overhead (PCB save, load)

Interval timer & interrupting clocking
 prevent a monopoly of system

Deadline Scheduling
 pay remnant cost for processing in explicit time
5.3 Scheduling Algorithms
1. FIFO(FCFS)
Finish
C
B
A
CPU
 Nonpreemptive
Ex) Job
1
2
3
Burst Time
24
3
3
 If arriving order  1, 2, 3
Job1
Job2 Job3
24
27
30
Average Turnaround Time = (24 + 27 + 30)/3 = 27
Average Waiting Time = (0 + 24 + 27)/3 = 17 ms
 If arriving order is 2, 3, 1
A.T.T = (3 + 6 + 30) / 3 = 13
A.W.T = (0 + 3 + 6) / 3 = 3
5.3 Scheduling Algorithms
2. Round Robin Scheduling for Timesharing System
Time Quantum of Time Slice (10 ~ 100 msec)
Preemptive
New job
Finish
Ex) In the case of jobs in previous example, if time quantum is 4,
Average turnaround time is -J1
J2
4
J3
7
J1
10
J1
14
·····
J1
47 / 3  16
30
Average Waiting Time = (4 + 7 + (10 - 4)) / 3 = 17 / 3 = 5.66 ms
 R.R Scheduling = Preemptive Scheduling
Time Quantum : if large
FCFS
short
like processor sharing
context switching overhead
5.3 Scheduling Algorithms
Process time = 10
Quantum
0
12
0
6
1
1
9
10
0
0
Context switches
6
1
2
3
4
5
6 7
10
8
9
10
Showing how a smaller time quantum increases context switching
Average turnaround time
5.3 Scheduling Algorithms
12.5
Process time
12.0
11.5
11.0
10.5
10.0
P1
6
P2
3
P3
1
P4
7
9.5
9.0
1
2
3
4
5
6
7
Time quantum
Showing how turnaround time varies with the time quantum
5.3 Scheduling Algorithms
3. Shortest - Job - First (SJF)
 Non preemptive Scheduling
 Long - Term Scheduling in Batch system : good.
 Short-Term Scheduling : impossible
Job
1
2
3
4
Burst Time
6
8
7
3

J4
0
J1
3
J3
9
J2
16
24
A.W.T = (3 + 16 + 9 + 0)/4 = 7 ms.
– FCFS A.W.T = (0 + 6 + 14 + 21)/4 = 41/4 = 10.25 ms.
– AVG Turnaround Time = (3 + 9 + 16 + 24)/4 = 52/4 = 13 ms.
5.3 Scheduling Algorithms
4. Shortest-Remaining-Time-First(SRT)
 Transformation of Shortest-Job-First
 Preemptive SJF
Ex)
Job
1
2
3
4
Arrive Time
0
1
2
3
J1 J2
0
1
J4
5

Burst Time
8
4
9
5
J1
10
* Assign the Thrasholded Value
J3
17
26
A.W.T = ((10-1) + (1-1) + (17 -2) + (5-3))/4 = 26/4 = 6.5 ms.
– A.T.T = 17 + (5-1) + (26-2) + (10-3) =52/4 = 13 ms.
– SJF A.T.T = 14.25 ms.
– SJF A.W.T = 5.75 ms.
5.3 Scheduling Algorithms
5. Priority Scheduling
 SJF : Special case of the general priority scheduling.
 FCFS : Equal priority.
Ex)
Job
1
2
3
4
5
J2
0
Burst Time
10
1
2
1
5
J5
1
Priority
3
1
3
4
2
J1
6
J3
16
J4
18
19
A.W.T = 8.2 ms
A.T.T = 12 ms
 Major problem : indefinite blocking or starvation.

Solution  HRN
5.3 Scheduling Algorithms
6. HRN(Highest-Response-ratio-Next) : Brinch Hansen
 priority = (Waiting Time + Service Time) / Service Time
 Nonpreemptive method that apply process service time and
total time to wait service
5.3 Scheduling Algorithms
7. Multilevel Queue
 processes each job classified into queues
 ex) classify into Foreground and Background (80% : 20%)


F.G is scheduled by Round Robin algorithm
B.G is scheduled by an FCFS algorithm
highest priority
system processes
Interactive processes
Interactive editing processes
batch processes
lowest priority
student processes
Multilevel Queue scheduling
5.3 Scheduling Algorithms
8. Multilevel Feedback Queue
 allow a process to move between queues
quantum = 8
quantum = 16
FCFS
Fig 5.7 Multilevel feedback queues
5.5 Real-Time Scheduling

Hard real-time
 within a guaranteed time

soft real-time
 less restrictive
5.6 Algorithm Evaluation

Deterministic modeling
Process
Burst Time
P1
10
P2
1
P3
2
P4
1
P5
5
 maximize CPU utilization
 maximize throughput)

Queueing Models
 result mathematical formula that estimate the distribution of
CPU and I/O bursts.

Simulations
5.6 Algorithm Evaluation
FCFS
R•R (Quantum=1)
S.J.F

Evaluation method
 Analytical evaluation


use mathematical analysis by predetermined workload, …
ex) Deterministic modeling
 Queuing Model

mathematical formula, arrival rate, service rate
utilization, queue length, wait time
 Simulation