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1Advanced Operating Systems
COMP9242
Introduction
2COMP9242 04S2
Staff
• Lecturer in Charge
– Gernot Heiser
• Lecturer 
– Kevin Elphinstone
• Various Support Staff
– TBA 
3COMP9242 04S2
Why are you here?
• You’ve done comp3231
– Did well (minimum credit)
– You would like to delve deeper into OS issues
– You’d like to get your hands really dirty
• Curious about where field is heading.
• Thinking about doing a thesis in OS
• Thinking about a job in embedded 
systems industry
4COMP9242 04S2
What can you expect?
• Challenging Project
• Lectures in general:
– Background required for project
– Exposure to local research projects
– An in-depth look at OS issues
• Building upon the background in COMP3231
– Exposure to recent and seminal research papers
– Guest lectures by active researchers (PhD students 
and local researchers)
5COMP9242 04S2
Project Goal
Provide students with a deeper 
understanding of Operating Systems 
through practical experience.
• Approach: Participate in the design and 
implementation of a simple operating system 
(SOS).
6COMP9242 04S2
Project Aims
• Provide experience in OS design and 
development, including:
– Microkernel based systems (L4::Pistachio).
• User-level OS servers.
• User-level page fault handlers.
– Device drivers
– Performance evaluation
– Implications of cache architectures
– Exposure to alternative OS Designs
• Demonstrate the importance of design
• Provide experience of being a team member in a 
large software project.
27COMP9242 04S2
Project Aims
• Expose students to a mostly realistic OS 
development environment.
– Similar to professional OS and or embedded systems 
developer. 
• Give an understanding of what’s involved in 
constructing an entire OS on bare hardware.
• Give an understanding of the interaction 
between low-level software and hardware. 
• Encourage you to undertake a thesis, or do 
research within OS Group / NICTA.
8COMP9242 04S2
Prerequisites
• Students are expected to be very 
competent C programmers.
• Students are expected to be familiar with 
– basic computer architecture concepts.
– Assembly language (read-only)
– Basic RISC processor characteristics (we’ll 
use a MIPS R4600 for the project)
9COMP9242 04S2
Lectures
(subject to change)
• Introduction and Overview
• Introduction to the L4 Microkernel
– L4 system calls and usage (to get you started on the 
project) 
• A close look at selected OS issues
– Protection, capabilities 
– Caching, and its implications for OS 
– Page tables for wide address spaces 
– SMP issues: locking, cache coherence, scheduling 
– File systems 
10COMP9242 04S2
Lectures
(subject to change)
• Microkernels and User-level Servers
– History and motivation for microkernel systems, 
Hydra, Mach, discussion, experiences; second-
generation microkernel systems, L4, Exokernel, Spin; 
design and implementation of microkernel-based 
systems, including user-level page fault handling and 
device drivers 
• Microkernel Implementation
– Detailed look at a real microkernel (L4Ka::Pistachio). 
• Persistent and single-address-space systems
• OS Projects at UNSW/NICTA
– Gelato@UNSW, NICTA Embedded OS, Mungi
11COMP9242 04S2
Project/Lab Work
• Build a simple operating system (SOS) from the 
ground up
• Major component of the course
• Use L4Ka::Pistachio 
– ported to MIPS here by Carl van Schaik
• Develop and test on U4600 computers
– R4600 based machines design and built by Kevin 
Elphinstone and Dave Johnson
– “Clean” machine to get your hands dirty
• Can also use CPU simulator Sulima
– Developed locally
– Demos must be on real hardware
12COMP9242 04S2
Project
• Some warm-up experiments
• Students will work in groups of two
• End goal:
– To produce a small efficient operating system
• Project will have a series of due milestones
– Demo to pass the milestone and be awarded marks
– Help you manage your time
– Avoid major problems
313COMP9242 04S2
Milestones
• Details on class web site
• Late milestones:
– Max 1 week late:  25% of milestone mark lost
– 1-2 weeks late: 50% of milestone mark lost
– More than two weeks: all marks lost
• Bonus task
– Much pain for little gain ;-)
– Don’t go overboard
– Full mark doesn’t require bonus!
14COMP9242 04S2
Alternative Projects
• Special arrangements might be made for 
particular student to do alternative projects
– Must be at least as challenging as the original 
project
– Must have some relevance to you or us.
– Must convince us that you can actually do it. 
– Unable to get bonus marks
15COMP9242 04S2
Assessment
• 65% for project work
• 35% for final exam
– A minimum of 14 (40%) required in final exam 
to pass
• Final Exam
– 24hr take-home exam
– Read and analyse two recent research papers 
and submit a critical report
16COMP9242 04S2
Textbook
• No particular textbook for course
– See course web page for useful reference 
books 
• Selected research papers referred to in 
the course
• Manuals provided in hardcopy and via 
class web site
L4 and Microkernels
Background
18COMP9242 04S2HW
Bit   Byte   Word    Register    
Instructions
Application
documents
windows
symbols
stacks & heaps
arrays & 
structures
variables
threads
coroutines
modules
procedures
statements

	





	

 
	



	
	

	


	

419COMP9242 04S2
Monolithic Kernels - Advantages
• Kernel has access to everything, 
potentially:
– All optimisations are possible
– All techniques/mechanisms/concepts are 
implementable
• Can be extended by simply adding more 
code to the kernel
20COMP9242 04S2
Linux Kernel Evolution
Linux Kernel Size (.tar.gz)
0
5
10
15
20
25
30
35
31-Jan-93 28-Oct-95 24-Jul-98 19-Apr-01 14-Jan-04
Date
Si
ze
 
(M
b)
 !
"#$#%&'"#(
 

21COMP9242 04S2
Approaches to tackling 
complexity
• Monolithic approaches
– Layered Kernels
– Modular Kernels
– Object Oriented Kernels 
• Alternatives
– Extensible Kernels
– Microkernels
22COMP9242 04S2
HW
Bit   Byte   Word    Register    
Instructions
µ-kernel
Address Space          Thread
Application
documents
windows
symbols
stacks & heaps
arrays & 
structures
variables
threads
coroutines
modules
procedures
statements
Server
File 
23COMP9242 04S2
History
• monolithic kernels
24COMP9242 04S2
History
• monolithic kernels
• 1st- generation µ-kernels
– Mach  
– Chorus      
– Amoeba     
– (L3)           
External Pager
User-Level Driver
CMU, OSF
Inria, Chorus
Vrije Universiteit
GMD
525COMP9242 04S2
Brief History
• monolithic kernels
• 1st- generation µ-kernels
– Mach  
– Chorus      
– Amoeba     
– (L3)           
• 2nd- generation µ-kernels
– (Spin)          
– Exokernel
– L4                
CMU, OSF
Inria, Chorus
Vrije Universiteit
GMD
U Washington
MIT
GMD / IBM / UKa
User-Level Address Space
External Pager
User-Level Driver
26COMP9242 04S2
HW
L4
classic
OS
Security
RT
MM
classic +
HW
L4
native
Java
em-
bedded
app
thin
HW
L4
highly-specialized
component
specialized
27COMP9242 04S2
HW
L4
SOS
Security
sosh
Project
sosh
28COMP9242 04S2
The Great Promise
• coexistence of different
• APIs
• file systems
• OS personalities
• flexibility
• extensibility
• simplicity
• maintainability
• security
• safety
29COMP9242 04S2
The Great PromiseThe Big Disaster
 
 	

 

• coexistence of different
• APIs
• file systems
• OS personalities
• flexibility
• extensibility
• simplicity
• maintainability
• security
• safety
Macro µ
6Performance                                 Flexibilty
Simplicity
rf r                                  l i ilit
Macro µ The 100-µs 
Disaster
25 MHz 386                  50 MHz 486                90 MHz Pentium             133 MHz Alpha
The 100-µs 
Disaster
34COMP9242 04S2
0
100
200
300
400
0 2000 4000 6000
msg len
Mach
LN  +  cache flush
LN
raw move
[µs]
µs / ipcIPC Costs (486, 50 MHz)
35COMP9242 04S2
0
50
100
0 50 100 150
msg len
[µs
]
Mach
LN
0
100
200
300
400
0 2000 4000 6000
msg len
Mach
LN  +  cache flush
LN
raw move
[µs]
IPC Costs (486, 50 MHz)
36COMP9242 04S2
L4 IPC
0.73 µs (Pentium 166 MHz)
0.91 µs (R4600 100 MHz)
0.10 µs (21164  433 MHz)
[cycles]
236
180
20
0
50
100
150
200
250
Pent III P3 Sysops P3 Lipc ?
0.47 µs (P III 500 MHz)
0.36 µs (P III 500 MHz)
0.04 µs (P III 500 MHz)
(hopefully)
82
16
23
36
55
7
38
0
20
40
60
80
100
120
140
Pentium R4600 Alpha
737COMP9242 04S2
Minimality
Elegance
Architectural 
Integration
Efficiency
Flexibility
• A µ-Kernel does the Job
• if Properly Designed
• if Carefully Implemented
Thesis:
38COMP9242 04S2
When analyzing IPC 
performance,
Cycles are not the only the to 
consider!!
39COMP9242 04S2
Processor-DRAM Gap 
(latency)
µProc
60%/yr.
DRAM
7%/yr.
1
10
100
1000
19
80
19
81
19
83
19
84
19
85
19
86
19
87
19
88
19
89
19
90
19
91
19
92
19
93
19
94
19
95
19
96
19
97
19
98
19
99
20
00
DRAM
CPU
19
82
Processor-Memory
Performance Gap:
(grows 50% / year)
Pe
rfo
rm
an
ce
Time
“Moore’s Law”

 )

		*
+&
40COMP9242 04S2
Today’s Situation: 
Microprocessor 
• Microprocessor-DRAM performance gap
– time of a full cache miss in instructions executed
1st  Alpha (7000): 340 ns/5.0 ns = 68 clks x 2 or 136
2nd Alpha (8400): 266 ns/3.3 ns = 80 clks x 4 or 320
3rd Alpha (t.b.d.): 180 ns/1.7 ns =108 clks x 6 or 648
– 1/2X latency x 3X clock rate x 3X Instr/clock  5X

 )

		*
 +&
41COMP9242 04S2
99.85%
0.15%
L2 cache
• 8192 cache lines (256K)
• 12 lines used for IPC
98.83%
1.17%
L1 cache
• 1024 cache lines (16K + 16K)
• 12 lines used for IPC
Cache Working Sets
42COMP9242 04S2
Other Complications
• P4 trace cache
– A cache of recently translated µ-ops
– Flushed on every page-table switch
• Virtual Caches
– Need to be flushed on address space switch
843COMP9242 04S2
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
100 200 400 800 1600 3200 6400 12800 25600 51200 102400 204800
Mach 486
Mach Alpha
Chorus 486
Spin Alpha
L4 Pentium
average cycles between successive IPCs
o
v
er
he
ad
 
du
e 
to
 
IP
C
44COMP9242 04S2
Α µ-kernel does no real work.
µ-Kernel services are only required to overcome µ-kernel constraints.
Therefore, µ-kernels have to be infinitely fast!
Minimality is the key!
• Threads
• Address Spaces
IPC
Mapping
45COMP9242 04S2
Threads, CommunicationAddress
Spaces
Threads
46COMP9242 04S2
Drivers at User Level
User
Driver
Device
INTR
=  ipc
• IO ports:  part of the user address space 
• interrupts:  messages from hardware
47COMP9242 04S2
Address Spaces
48COMP9242 04S2
Address Spaces
• map
• unmap
• grant
949COMP9242 04S2
Address Spaces
• map
• unmap
• grant
50COMP9242 04S2
Address Spaces
• map
• unmap
• grant
51COMP9242 04S2
Page Fault Handling
Pager
"PF" msg
map msg
Application
52COMP9242 04S2
Page Fault Handling
Pager
"PF" msg
map msg
Application
PF     IPC
res IPC
53COMP9242 04S2
Address Spaces
Physical Memory
54COMP9242 04S2
Address Spaces
Physical Memory
Initial AS
10
55COMP9242 04S2
Address Spaces
Physical Memory
Initial AS
Pager 1 Pager 2
56COMP9242 04S2
Address Spaces
Physical Memory
Initial AS
Pager 1 Pager 2
Pager 3
Pager 4
57COMP9242 04S2
Address Spaces
Physical Memory
Initial AS
Pager 1 Pager 2
Pager 3
Pager 4
Application
Application Application
Application
58COMP9242 04S2
Address Spaces
Physical Memory
Initial AS
Pager 1 Pager 2
Pager 3
Pager 4
Application
Application Application
Application
Driver
Driver
59COMP9242 04S2
Mach Virtual Memory
In comparison
Physical Memory
Paging Policy

Application Application
Application
External Pager ,
60COMP9242 04S2
Size Comparison
-	 .!
"#(
$&/!
+0*000
$1!!
	23"
%0*000
11
61COMP9242 04S2
HW
LN
classic
OS
Security
RT
MM
classic +
HW
LN
native
Java
em-
bedded
app
thin
HW
LN
highly-specialized
component
specialized