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Duke Systems
CPS 210
Software Architecture and OS Platforms
Jeff Chase
Duke University
http://www.cs.duke.edu/~chase/cps210
Friday
• Recitations
– Fridays: 3:05-4:20 PM
– Soc Sci 139: see maps.duke.edu
– TA: Vamsi Thummala
– This week: important info for P1A: heap manager
• Monday
– Prof. Landon Cox on Unix
– Reading: flashback to 1968
“Software Architecture”
User Applications
Software
architecture
Physics stops here.
Operating System(s)
Computer
architecture
Substrate / Architecture
Comparative architecture: what works
Reusable / recurring design patterns
• Used in OS
• Supported by OS
Abstraction(s)
• A means to organize knowledge
– Capture what is common and essential
– Generalize and abstract away the details
– Specialize as needed
– Concept hierarchy
• A design pattern or element
– Templates for building blocks
– Instantiate as needed
• E.g.: class, subclass, and instance
Platform abstractions
• Platforms provide “building blocks”…
• …and APIs to use them.
– Instantiate/create/allocate
– Manipulate/configure
– Attach/detach
– Combine in uniform ways
– Release/destroy
• Abstractions are layered.
– What to expose? What to hide?
The choice of abstractions reflects a philosophy
of how to build and organize software systems.
[Garlan/Shaw 94]
Components and connectors
Abstract Data Type (ADT)
object/class/module
Connector
pipes/filters
objects / ADTs
events
layering
repository
interpreter
Component
Component
Garlan and Shaw, An Introduction to Software Architecture, 1994.
“Components in context”
Context
(Domain)
Thread
Program
For our purposes, an operating system is a platform that supports
protection and isolation: every component runs within a context.
Program, context and thread are OS abstractions.
Isolated contexts (domains)
On most platforms, contexts are isolated.
They cannot interfere with one another, even if
they are running the same program.
I.e., each context is its own “sandbox”.
The programs may fail independently.
In classical OS, a context is called a process.
Each has a private virtual address space.
Butler Lampson’s definition: “I am
isolated if anything that goes wrong is
my fault (or my program’s fault).”
Isolation might not be absolute.
E.g., the program instances may choose to
interact, or may access shared data.
Running a program
code
constants
initialized data
imports/exports
symbols
types/interfaces
data
Program
When a program launches, the OS platform
allocates memory to store its code and data.
It may establish a new context and/or thread.
The Operating System
• An operating system:
– sets up the contexts
– enforces isolation
– mediates interactions
• Applications trust the OS to do these things.
• Any software platform for running
application programs can be called an OS.
The Trusted Computing Base (TCB)
• Programs may incorporate (link to) libraries packaged with the
OS or written by third parties.
• These libraries may even define a system API that applications
use. (e.g., heap manager malloc and free).
• But they run within the program’s context, and so are not
isolated from the rest of the program at runtime. Any failure in
the library can damage the program, and any failure of the rest
of the program can damage the library.
• These libraries have no more privilege or power than any other
part of the program.
• The part of a platform that is responsible for the integrity of
application contexts is called its TCB.
• Trusted software is often called the kernel in classical OS.
Operating Systems: The Classical View
Programs
run as
independent
processes.
data
data
Protected
system calls
Protected OS
kernel
mediates
access to
shared
resources.
Each process
has a private
virtual address
space and one
or more
threads.
...and upcalls
(e.g., signals)
Threads
enter the
kernel for
OS
services.
The kernel code and data are protected from untrusted processes.
Key Concepts for Classical OS
•
kernel
• The software component that controls the hardware
directly, and implements the core privileged OS functions.
• Modern hardware has features that allow the OS kernel to
protect itself from untrusted user code.
•
thread
• An executing instruction path and its CPU register state.
•
virtual address space
• An execution context for thread(s) defining a name space
for executing instructions to address data and code.
•
process
• An execution of a program, consisting of a virtual address
space, one or more threads, and some OS kernel state.
Protection Systems 101
Reference monitor
Example: OS platform
Isolation boundary
Principles of Computer System Design  Saltzer & Kaashoek 2009
Post-note
•
This slide introduces some new terminology that we will hear more
about later:
•
Principal. Some entity that requests an action. It is often (but not
always) associated with a real-world identity, such as a user or
account. E.g., in Android, each application is a separate principal.
•
Authentication. When data arrives through a channel, or a channel is
established, we generally need to determine something about the
principal controlling the other endpoint of the channel. I.e., “to whom
am I speaking?”. (See slides on “channels” later in this deck.)
•
Authorization. Checks to determine if a given authenticated caller (a
subject or principal) is permitted to perform the requested operation,
according to an access control policy or procedure (the guard).
•
Reference monitor. Any component that has an API that is externally
visible (through a channel) must check authorization for operations on
that API before performing the op. The component must be isolated
from its callers, so it must run in a context that is distinct from the
callers. We can use the term reference monitor to refer to the context,
or the component, or the guard.
Threads: a familiar metaphor
1
Page links and
back button
navigate a
“stack” of pages
in each tab.
2 Each tab has its own stack.
One tab is active at any given time.
You create/destroy tabs as needed.
You switch between tabs at your whim.
3
Similarly, each thread has a separate stack.
The OS switches between threads at its whim.
One thread is active per CPU core at any given time.
time 
OS Platform: A Better Model
Applications
Libraries: shared by
multiple applications
Platform: same for all
applications
OS platform mediates access to shared resources.
[RAD Lab]
[Garlan/Shaw 94]
Components and connectors
Abstract Data Type (ADT)
object/class/module
Connector
pipes/filters
objects / ADTs
events
layering
repository
interpreter
Component
Component
Garlan and Shaw, An Introduction to Software Architecture, 1994.
GS1. Pipes and filters
Garlan and Shaw, An Introduction to Software Architecture, 1994.
GS2. Objects and invocation
Garlan and Shaw, An Introduction to Software Architecture, 1994.
GS3. Events and implicit invocation
publish
notify
subscribe
register
Event producers {publish/raise/announce/signal} events to notify
{consumers/receivers/subscribers} of occurrences relating to a
{subject/topic/category}.
The system invokes a registered handler method/procedure in each receiver.
Delivery is synchronous or asynchronous.
Communication: endpoints and channels
endpoint
port
channel
pipe
binding
connection
data transfer
stream
flow
messages
request/reply
RPC
events
operations
advertise (bind)
listen
connect (bind)
close
write/send
read/receive
If one side advertises a named
endpoint, we call it a server.
If one side initiates a
channel to a named endpoint,
we call it a client.
Pipes
parent
operations
close
write
read
pipe
Unix supports IPC channels called pipes between pairs of processes.
 Unidirectional byte stream (upstream, downstream).
 Brokered by a common parent that creates the pair.
 The child processes need not create/initiate the pipe or even
know where it comes from or where it goes.
 A pipe may be the “standard” input or output.
Example: Unix Pipes
[http://www.bell-labs.com/history/unix/philosophy.html]
MapReduce/Hadoop
MapReduce is a filter-and-pipe model for
data-intensive cluster computing
We stopped here
We discussed the need for protected contexts (“sandboxes”), even on
single-user systems like your smartphone. The discussion touched on
various dimensions of isolation for protected contexts (e.g., processes):
•Fault isolation. One app or app instance (context or process) can fail
independently of others. If it fails, the OS can kill the process and reclaim
all of its memory, etc.
•Performance isolation. The OS manages resources (“metal and glass”:
computing power, memory, disk space, I/O throughput capacity, network
capacity, etc.). Each instance needs the “right amount” of resources to
run properly. The OS prevents apps from impacting the performance of
other apps. E.g., the OS can prevent a program with an endless loop
from monopolizing the processor.
•Security. An app may contain malware that tries to corrupt the system,
steal data, or otherwise compromise the integrity of the system. The OS
uses protected contexts and a reference monitor to check and authorize
all accesses to data or objects outside of the context, including channel
setup.)
OS protection
The discussion touched on how a classical OS uses the hardware
to protect itself. (More on this later.)
• In particular, machines support events. The OS kernel
registers handlers for various machine events when it boots
(starts up). These events include machine exceptions (faults),
which may be caused by errant code, interrupts from the clock
or external devices (e.g., network packet arrives), and
deliberate kernel calls (traps) caused by programs requesting
service from the kernel through its API.
• All of these machine events make safe control transfers into
the registered kernel handler for the named event. Applications
run in sandboxes that prevent them from calling procedures in
the kernel or accessing kernel data directly (unless the kernel
chooses to allow it).
• In fact, after the system is booted, these events are the only
ways to ever enter the kernel, i.e., to run code in the kernel.
GS4. Layered systems
Garlan and Shaw, An Introduction to Software Architecture, 1994.
Post-note
• Note that an operating system is a layered system.
– The platform API encapsulates all the functionality below.
– Other platforms may be layered on top (“nested dolls”)
– Applications above the platform API provide other useful
functions.
– These application-layer programs include utilities (e.g.,
program development tools) and servers (e.g., window
system).
– See the layering diagrams for Android and Unix.
• Also, an OS uses layering below the platform API.
– e.g., network protocol stack (later)
– device driver software, etc.
Unix defines uniform, modular ways to combine
programs to build up more complex functionality.
Other application programs
sh
nroff
who
cpp
a.out
Kernel
date
comp
Hardware
cc
wc
as
ld
grep
vi
ed
Other application programs
GS5. Shared state repository
Garlan and Shaw, An Introduction to Software Architecture, 1994.
State
storage object
segment
file
table
object
access
attach/detach
mount/unmount
map/unmap
open/close
put/get
fetch
read/write
push
commit
store
filesystem
repository
volume
database
target
bit, byte
block
page
row
fragment
stripe
object
GS6. Interpreter
Garlan and Shaw, An Introduction to Software Architecture, 1994.
Interpreter: example
An interpreter controls
how a program executes
and what it sees.
An interpreter can
“sandbox” a program
for isolation.
ECMAscript
Some languages and interpreters are safer than others.
Architectural styles: tradeoffs
• constraints on composition
– give rise to general properties of compositions
• generality of connectors
– data type signatures
• coupling and control
• concurrency
• state (data) partitioning and sharing
– less sharing  better scalability
– standardizing interfaces vs. schema
The Birth of a Program (C/Ux)
myprogram.c
int j;
char* s = “hello\n”;
myprogram.o
assembler
data
object
file
int p() {
j = write(1, s, 6);
return(j);
}
data
data
data
…..
p:
compiler
libraries
and other
objects
linker
store this
store that
push
jsr _write
ret
etc.
myprogram.s
data
program
myprogram
(executable file)