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Slide 6-1
6
Threads and
Scheduling
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Announcements
• Program Assignment #1 due Tuesday Feb.
15 at 11 am
– TA will explain parts b-d in recitation
• Read chapters 6 and 7
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-2
Scheduling
Slide 6-3
• A process can be switched out due to:
–
–
–
–
blocking on I/O
voluntarily yielding the CPU
being preemptive time sliced, i.e interrupted
termination
• The OS’s scheduler implements a scheduling policy that
selects the next process to run from the ready queue, based
on several criteria
–
–
–
–
–
CPU utilization - 40% to 90%
Throughput - # processes completed/second
Turnaround Time - how long it takes to execute that process
Waiting Time - sum of time waiting in the ready queue
Response Time - time until first response
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Scheduling
Slide 6-4
• The dispatcher gives control of the CPU to
the process selected by the scheduler, which
involves
– switching context
– switching to user mode
– jumping to the proper location in the user
program to restart that program
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-5
FCFS Scheduling
• First Come First Serve:
order of arrival dictates
order of scheduling
• If processes arrive in
order P1, P2, P3, then
Gantt chart of CPU
service time is:
Process
P1
CPU
Service
Time
24
P2
3
P3
3
P1
0
Copyright © 2004 Pearson Education, Inc.
P2
24
P3
27
Operating Systems: A Modern Perspective, Chapter 6
30
Slide 6-6
FCFS Scheduling
Process
• If processes arrive in
reverse order P3, P2,
P1, then Gantt chart of P1
CPU service time is: P2
P3
P2
0
3
P3
3
3
P1
6
Copyright © 2004 Pearson Education, Inc.
CPU
Service
Time
24
30
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-7
FCFS Scheduling
P1
P2
0
24
P2
0
3
P3
P3
27
30
P1
6
30
• average wait time is (0+24+27)/3 = 17 seconds
in top scenario
• average wait time is (0+3+6)/3 = 3 seconds in
2nd scenario
• FCFS wait times are generally not minimal,
and vary if process service times vary a lot
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
FCFS Scheduling
• If there is one CPU-bound process, and
many I/O bound processes, then
– convoy effect: I/O bound processes finish their
I/O and wait for one large process to complete,
then execute quickly, block on I/O, while CPUbound process dominates again
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-8
Shortest Job First Scheduling
• Choose the process/thread with
the lowest service time
– gives priority to shortest processes
– minimizes the average wait time
• can prove this - out of 24
possibilities, this has the lowest
average wait time
• intuition: moving a short process
before a long one decreases the wait
time of short processes more than it
increases the wait time of long
processes
P4
0 3
Copyright © 2004 Pearson Education, Inc.
P1
P3
9
Process
P1
P2
P3
P4
P2
16
24
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-9
CPU
Service
Time
6
8
7
3
av wait time
= (0+3+9+16)/4
= 7 seconds
Shortest Job First Scheduling
Slide 6-10
• It is difficult to know the length of each job a priori
– difficult to know the length of each CPU burst of a job
• So predict the burst length of each job/process/thread
– for each thread, track its pattern of CPU usage, and keep a running
average
• could be a sliding window
• instead, this is often an exponentially weighted average
– Ave(n) = (1-a)*CPU_usage(n) + a*Ave(n-1), where 0<a<1
– This can be rewritten Ave(n) = S (1-a)* an-i-1 * CPU_usage(i) summed
from i=0 to n-1
• Can be preemptive, i.e. when a new job arrives in ready
queue, if its service time is less than currently executing
job’s service time, then it preempts current job
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-11
Priority Scheduling
• SJF is a special case of priority
scheduling, where priority is given
to short jobs
• Any criteria can be used to decide
on a priority
– measurable characteristics of the
process
– external criteria
• Can be preemptive, i.e. higher
priority process arrives in the
ready queue and preempts a lower
priority process
P2
P5
0 1
Copyright © 2004 Pearson Education, Inc.
P1
6
Process CPU
Service
Time
P1
10
P2
1
P3
2
P4
1
P5
5
P3 P4
16 18 19
Operating Systems: A Modern Perspective, Chapter 6
Priority
3
1
4
5
2
Priority Scheduling
• Can starve low priority processes
• A solution is aging: as low priority
processes age, their priority is increased
– if priorities range from 1-128, can increase
(decrement) the priority by 1 every T seconds
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-12
Slide 6-13
Deadline Scheduling
• Hard real time systems
require that certain processes
must complete execution
within a certain time, or the
system crashes
Process
P0
CPU
Service
Time
350
Deadline
from
now
575
P1
125
550
P2
475
1050
P3
250
none
P4
75
200
– robots
• Admission control policy
– No notion of not being able
to admit a process in FCFS,
SJF, and priority
– Here, can’t admit a process
if its deadline can’t be met
– This is typical of many
Quality-of-Service
scheduling policies in
networking - can’t admit a
new stream/flow of packets
if its QOS
deadlines/guarantees cannot
be met
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-14
Deadline Scheduling
i
0
1
2
3
4
t(pi) Deadline
350
575
125
550
475 1050
250 (none)
75
200
•Allocates service by deadline
•May not be feasible
200
550 575
1050
0
1275
p1
p4
p4
Copyright © 2004 Pearson Education, Inc.
p4
p1
p0
p0
p2
p3
p0
p2
p3
p2
p3
p1
Operating Systems: A Modern Perspective, Chapter 6
Round Robin Scheduling
Slide 6-15
• The ready queue is treated as a circular queue, and the
CPU scheduler rotates among the processes in the ready
queue, giving each a time slice, after which it is preempted
by a timer interrupt and another process is started
– useful for time sharing multitasking systems - most widely used
scheduling algorithm
– combines FCFS and preemption
– simple and fair, though wait times can be long
• Fair: If there are n processes, each process gets 1/n of CPU
• Simple: Don’t need to know service times a priori
• A process can finish before its time slice is up. The
scheduler just selects the next process in the queue
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-16
Round Robin Scheduling
• Suppose we use a time slice of 4
Process CPU
ms, and ignoring context switch
Service
overhead
Time
• Now P1 is time sliced out, and P2
(ms)
and P3 are allowed to run sooner
P1
24
than FCFS
• average wait time is 5.66 ms
P2
3
P3
3
P1
0
P2
4
Copyright © 2004 Pearson Education, Inc.
P2
7
10
P1
P1
14
P1
18
P1
22
Operating Systems: A Modern Perspective, Chapter 6
P1
26
30
Round Robin Scheduling
Slide 6-17
• Context switch overhead affects the choice of time
slice
– context switches can take 10 microseconds to copy
register state to/from memory
• on a 1 GHz CPU, that’s 10000 wasted cycles per context
switch!
– if the time slice is on the order of a context switch, then
CPU spends most of its time context switching
– Typically choose time slice to be large enough so that
only 10% of CPU time is spent context switching
– Most modern systems choose time slices of 10-100 ms
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Round Robin Scheduling
• Weighted Round Robin - each process is
given some number of time slices, not just
one per round
• this is a way to implement priorities with
preemptive time slicing
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-18
Multi-level Queue Scheduling
• Partitions ready queue into several queues
– different processes have different needs, e.g.
foreground and background
– so don’t apply the same scheduling policy to
every process, e.g. foreground gets RR,
background gets FCFS
• Queues can be organized by priority, or
each given a percentage of CPU, or hybrid
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6
Slide 6-19
Multi-level Feedback Queues
Slide 6-20
• Allows processes to move between queues
• Criteria for movement depends
– could be aging
– could be CPU-bound processes move down the
hierarchy of queues, allowing interactive and I/O-bound
processes to move up
• give a time slice to each queue, with smaller time slices higher
up
• if a process doesn’t finish by its time slice, it is moved down to
the next lowest queue
Copyright © 2004 Pearson Education, Inc.
Operating Systems: A Modern Perspective, Chapter 6