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Lectures
Lecture 16 Template
16 - Scheduling (3)
Outline
Priority Scheduling
Announcements
Reading:
Priority Scheduling
Processes get assigned a priority (weight).
Example 1:
Multiple Queues
Other Algorithms
PA 3 instructions are out, a few things are still
missing that will hopefully be out be end of week
Quiz 2 this Friday
1)
2)
3)
MOS 2.4
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Always finish highest priority jobs first
Each priority level has an associated queue
Priority queue processed Round-Robin
Priority 1
Priority 2
Priority 3
Priority 4
Queue
Headers
Runnable
Processes
(Round-Robin)
MOS Figure 2-42
P1
P1
3
10
1
2
1
5
1
4
5
2
Priority
Processing
Time
P2
P2
P3
P3
P4
P4
P5
P5
higher priority = lower number
(Linux convention)
Turnaround Time:
Waiting Time:
queues:
1: P2
2: P5
3: P1
4: P3
5: P4
each queue must
finish processing
before the next
begins.
t = 0
ready:
1: P2
2: P5
3: P1
4: P3
5: P4
Quantum = 2
Example 2:
P1
3
10
1
2
1
5
1
3
3
2
Priority
Processing
Time
P2
P3
P4
P5
Quantum = 2
queues:
1: P2
2: P5
3: P1, P3, P4
t = 0
ready:
1: P2
2: P5
3: P1, P3, P4
P1
P2
P3
P4
P5
Turnaround Time:
Waiting Time:
Assigning Priorities
Static assignment by system admin:
Linux Priorities
Windows Priorities
Dynamic assignment by kernel algorithm
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Process type
Starvation possible
Priority inversion possible
User Role
kernel measures runtime statistics
numbered 0-139, lower number = higher priority
real-time class: 0-99; normal class: 100-139
kernel assigns default priority of 120
priority modified with `nice` modifier
process list commands show different numbers
for historical reasons
example: 1/f
process types defined into priority groups
processes assigned default priority
system admin can modify individual process priority
priority assigned based on user id/group ownership
e.g. based on QoS agreements between customer/admin
priority calculated based on process's past behavior
f is proportion of quantum used
longer a process runs for, the lower its priority
I/O bound tasks tend to higher priority for better interactivity
nice = -20 => priority 100 (admin only)
nice = +19 => priority 139
nice modifier value subject to imposed limits
`ps -elf` shows PRI = number - 40 = 80 + nice
`top` shows PRI = number - 100 = 20 + nice
Real-Time
Normal
0
99
100
139
priority
higher
lower
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32-level priority scheme, 6 different classes
higher number = higher priority
real-time class: 16-31; other classes 1-15
system idle thread priority 0
default: normal class, normal priority (8)
time-critical
real-time
31
15
15
15
10
15
15
15
14
13
12
12
11
10
9
9
8
7
7
6
5
5
4
3
2
4
6
6
8
8
1
1
1
1
11
1
26
25
24
23
22
16
high
above
normal
normal
below
normal
idle
highest
above normal
normal
below normal
lowest
idle
Multiple Queues
Similar to priority queues, but priority is not
assigned to process itself, only to queues.
Jobs start in highest priority queue and move to
lower queues after getting processing time.
Jobs move back to highest priority queue when
"interaction" occurs (as defined by kernel)
longer running jobs move to lower queues where
fewer interruptions occur, while highly interactive
ones stay in higher queues.
High responsiveness for I/O-bound jobs
CPU-bound processes have fewer interruptions
multiple queues need to be managed
SJF but kernel guesses processing times
N processes get 1/N total processing time
every process in ready queue given lottery
tickets, and kernel picks a ticket at random
starvation is possible but unlikely
give more tickets to increase priority
RR over users instead of processes
limit processing time per user regardless of
number of user processes
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higher queues processed RR before lower ones
each queue is assigned different time quantum
e.g. moving mouse into window
user can potentially abuse interactions
guarantees CPU time to all processes
Problems:
lower priority task blocks higher priority task if it locks a resource
needed by higher priority task
Priority 1
Q = 1
Priority 2
Q = 2
Priority 3
Q = 4
Priority 4
Q = 8
Queue
Headers
Runnable
Processes
(Round-Robin)
Example 2:
P1
0
50
2
10
0
8
Release
Time
Processing
Time
P2
P3
5 queues with quanta:
(1, 2, 4, 8, 16)
t = 0
queues:
1: P1, P2
2:
3:
4:
5:
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Other Algorithms
Shortest Process Next
Guaranteed Scheduling
Lottery Scheduling (probabilistic scheduling)
Fair Share Scheduling