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Design philosophy of operating systems (III)

Recap: impact of UNIX
Clean abstraction — everything as a file

File system — will discuss in detail after midterm
Portable OS
• Writteninhigh-levelCprogramminglanguage
• TheunshakablepositionofCprogramminglanguage
We are still using it!

Recap: Each process has a separate virtual memory space
static data
heap Virtual memory
static data
heap Virtual memory
static data
heap Virtual memory
static data
heap Virtual memory
They are isolated from one another. Each of them is not supposed to know what happens to another one
Virtually, every process seems to have a processor, but only a few of them are physically executing.

Recap: impact of UNIX
Clean abstraction — everything as a file
File system — will discuss in detail after midterm
Portable OS
• Writteninhigh-levelCprogramminglanguage
• TheunshakablepositionofCprogramminglanguage
We are still using it!

Recap: Review the first demo

UNIX Recap: Protection mechanisms
• Protectionisassociatedwitheachfile—describedinthemetadata
Each file contains three (only two in the original paper) types of
Each type of users can have read, write, execute permissions setuid to promote right amplifications

The basic process API of UNIX

What happens if we execute the following code?
int main() {
the parent’s PID is 2; child’s PID is 7.
if ((pid = fork()) == 0) {
} printf (”My pid is %d\n”, getpid());
printf (”Child pid is %d\n”, pid);
What will happen?
# of times “my pid” is printed
my pid values printed
# of times “child pid” is printed
child pid values printed

UNIX’s interface of managing processes

What happens if we add an exit?
int main() {
if ((pid = fork()) == 0) {
printf (”My pid is %d\n”, getpid());
} exit(0);
printf (”Child pid is %d\n”, pid);
return 0; }
the parent’s PID is 2; child’s PID is 7.
Ifweaddanexit…
# of times “my pid” is printed
my pid values printed
# of times “child pid” is printed
child pid values printed

fork() and exit()
if ((pid = fork()) == 0) {
printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
static data
heap stack
Virtual memory

fork() and exit()
if ((pid = fork()) == 0) {
printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
static data
heap stack
Virtual memory
if ((pid = fork()) == 0) {
printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
static data
heap stack
Virtual memory

fork() and exit()
Output: Mypidis7
if ((pid = fork()) == 0) {
printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
static data
heap stack
Virtual memory
if ((pid = fork()) == 0) {
printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
static data
heap stack
Virtual memory

fork() and exit()
Output: Mypidis7
if ((pid = fork()) == 0) {
printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
static data
heap stack
Virtual memory

fork() and exit()
Output: Mypidis7 Child pid is 7
if ((pid = fork()) == 0) {
printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
static data
heap stack
Virtual memory

What happens if we add an exit?
int main() {
if ((pid = fork()) == 0) {
printf (”My pid is %d\n”, getpid());
} exit(0);
printf (”Child pid is %d\n”, pid);
return 0; }
the parent’s PID is 2; child’s PID is 7.
# of times “my pid” is printed
Ifweaddanexit…
my pid values printed
# of times “child pid” is printed
child pid values printed

Poll close in
Consider the following code
printf(“moo\n”);
printf(“oink\n”);
printf(“baa\n”);
How many animal noises will be printed? A. 3
B. 6 C. 8 D. 14 E. 24
More forks

Consider the following code
fork(); 2x printf(“moo\n”);
printf(“oink\n”);
printf(“baa\n”);
How many animal noises will be printed? A. 3
More forks

Starting a new program with execvp() int execvp(char *prog, char *argv[])
What execvp does:
• Stopsthecurrentprocess
• Overwritesprocess’addressspaceforthenewprogram • Initializeshardwarecontextandargsforthenewprogram • Insertstheprocessintothereadyqueue
execvp does not create a new process 21
fork does not start a new program, just duplicates the current

Why separate fork() and exec()
Windows only has exec
Flexibility
Allows redirection & pipe
• Theshellforksanewprocesswheneveruserinvokeaprogram
• Afterfork,theshellcansetupanyappropriateenvironment
variable to before exec
The shell can easily redirect the output in shell: a.out > file

if ((pid = fork()) == 0) { execvp(“a.out”,NULL); printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
Virtual memory static data
heap stack

if ((pid = fork()) == 0) { execvp(“a.out”,NULL); printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
Virtual memory static data
heap stack
if ((pid = fork()) == 0) { execvp(“a.out”,NULL); printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
Virtual memory static data
heap stack

Output: Child pid is 7
if ((pid = fork()) == 0) { execvp(“a.out”,NULL); printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
Virtual memory static data
heap stack
if ((pid = fork()) == 0) { execvp(“a.out”,NULL); printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
Virtual memory static data
heap stack

Output: Child pid is 7
New program!
if ((pid = fork()) == 0) { execvp(“a.out”,NULL); printf(“My pid is %d\n”, getpid());
printf(“Child pid is %d\n”, pid);
Virtual memory static data
heap stack
int main() {
printf(“New program!”);
}return 0;
Virtual memory static data
heap stack

Poll close in
What’s in the kernel?
How many of the following UNIX features/functions are implemented
in the kernel?
! I/Odevicedrivers
” Filesystem
$ Virtualmemorymanagement A. 0
B. 1 C. 2 D. 3 E. 4

Let’s write our own shells

• • • • • • •
How to implement redirection in shell
Say, we want to do ./a > b.txt
The forked code opens b.txt
The forked code dup the file descriptor
The forked code assigns b.txt to stdin/stdout The forked code closes b.txt
exec(“./a”, NULL)

How to implement redirection in shell
Say, we want to do ./a > b.txt
• The forked code opens b.txt
Homework for you:
Think about the case when
your fork is equivalent to fork+exec()
The forked code dup the file descriptor to stdin/stdout The forked code closes b.txt
exec(“./a”, NULL)
int pid, fd;
char cmd[2048], prompt = “myshell$” while(gets(cmd) != NULL) {
if ((pid = fork()) == 0) {
fd = open(“b.txt”, O_RDWR | O_CREAT, S_IRUSR |
S_IWUSR); dup2(fd, stdout); close(fd);
}execv(“./a”,NULL);
} printf(“%s ”,pro
static data
The shell can respond to next in
heap stack
int pid, fd;
char cmd[2048], prompt = “myshell$” while(gets(cmd) != NULL) {
if ((pid = fork()) == 0) {
fd = open(“b.txt”, O_RDWR | O_CREAT, S_IRUSR |
S_IWUSR); dup2(fd, stdout); close(fd);
}execv(“./a”,NULL);
} printf(“%s ”,pro
static data
heap stack

pid_t wait(int *stat)
pid_t waitpid(pid_t pid, int *stat, int
wait / waitpid suspends process until a child process ends • wait resumes when any child ends
• waitpid resumes with child with pid ends
• exit status info 1 is stored in *stat
• Returnspidofchildthatended,or-1onerror
Unix requires a corresponding wait for every fork

What’s in the kernel?
How many of the following UNIX features/functions are implemented
in the kernel?
! I/Odevicedrivers
” Filesystem
$ Virtualmemorymanagement A. 0
user-level kernel
privilege boundary

A user program provides an interactive UI
Interprets user command into OS functions
Basic semantics:
command argument_1 argument_2 …
Advanced semantics
• Redirection •>
Multitasking

Clean abstraction The impact of UNIX
File system — will discuss in detail after midterm
Portable OS
• Writteninhigh-levelCprogramminglanguage
• TheunshakablepositionofCprogramminglanguage
We are still using it!

Mach: A New Kernel Foundation For UNIX
Development
, , , , , ,
Computer Science Department, University

Poll close in
Why is “Mach” proposed?
How many of the following statements is/are true regarding the motivations
of developing Mach in 1986?
! ModernUNIXsystemsdonotprovideconsistentinterfacesforsystemfacilities
” SystemlevelservicescanonlybeprovidedthroughfullyintegrationoftheUNIX kernel
# Theprocessabstractioncannotusemultiprocessorsefficiently $ Networkcommunicationisnotprotected

The cost of creating processes
Measure process creation overhead using lmbench http://
www.bitmover.com/lmbench/

The cost of creating processes
On a 3.2GHz intel Core i5-6500 Processor • Processfork+exit:53.5437microseconds
• Morethan16Kcycles
Measure process creation overhead using lmbench http://
www.bitmover.com/lmbench/

Why is “Mach” proposed?
How many of the following statements is/are true regarding the motivations
of developing Mach in 1986?
! ModernUNIXsystemsdonotprovideconsistentinterfacesforsystemfacilities
” SystemlevelservicescanonlybeprovidedthroughfullyintegrationoftheUNIX kernel
# Theprocessabstractioncannotusemultiprocessorsefficiently $ Networkcommunicationisnotprotected

The hardware is changing
• Multiprocessors
Why “Mach”?
• Networkedcomputing
The software
• ThedemandofextendinganOSeasily
• Repetitivebutconfusingmechanismsforsimilarstuffs
Make UNIX great again!

Poll close in
How many pairs of the “why” and the “what” in Mach are correct?
Whys v.s. whats
(1) (2) (3) (4)
Support for multiprocessors
Networked computing
OS Extensibility
Messages/Ports
Kernel debugger
Repetitive but confusing mechanisms Messages/Ports
A. 0 B. 1 C. 2 D. 3 E. 4

The hardware is changing
• Multiprocessors
Why “Mach”?
Networked computing
The software
• ThedemandofextendinganOSeasily
Repetitive but confusing mechanisms for similar stuffs

UNIX provides a variety of mechanisms • Pipes
No protection
No consistency Location dependent
Interprocess communication

Port is an abstraction of:
• Messagequeues
• Capability
What do ports/messages promote?
• Locationindependence—everythingiscommunicatingwithports/ messages, no matter where it is
Ports/Messages

Ports/Messages
Capability of Z Message queues
read, write
Capability of A
read, write
message = “something”;
send(port Z, message);
Capability of B
read, write
recv(port Z, message);

What is capability? — Hydra
An access control list associated with an object
Contains the following: • Areferencetoanobject • Alistofaccessrights
Whenever an operation is attempted:
• Therequestersuppliesacapabilityofreferencingtherequesting
The OS kernel examines the access rights
• Type-independant rights
• Type-dependent rights
object — like presenting the boarding pass

Poll close in
Tasks/Processes and threads
How many of the following regarding the comparison of parallelizing
computation tasks using processes and threads is/are correct?
! Thecontextswitchandcreationoverheadofprocessesishigher
” Theoverheadofexchangingdataamongdifferentcomputingtasksforthe same applications is higher in process model
# Thedemandofmemoryusageishigherwhenusingprocesses
$ Thesecurityandisolationguaranteesarebetterachievedusingprocesses

Intel Sandy Bridge
Share L3 $

Concept of chip multiprocessors
Core Core Registers Registers
L1-$ L1-$ L2-$ L2-$
Registers L2-$ L2-$
Last-level $ (LLC)
Main memory is eventually shared among processor
Main memory

Tasks/processes
PC CPU Memory
Each process has its own unique virtual memory address ace, it of execu set of I/O
s own states
tion, its own
static data
static data
heap Virtual memory
a = 0x01234567
heap Virtual memory
a = 0xDEADBEEF
static data
static data
heap Virtual memory
a = 0x87654321
heap Virtual memory
a = 0x95273310

Task#1 Thread #1 Thread #2
CPU PC CPU
Thread #3 Thread #1 Thread #2
CPU PC PC CPU PC CPU
Each process has its own unique virtual memory address
space, its own states of execution, its own set of I/Os
Each thread has its own PC, states of execution, but shares
memory address spaces, I/Os without threads within the
same process
static data
static data
heap Virtual memory
a = 0x01234567
heap Virtual memory
a = 0x01234567

Tasks/Processes and threads
How many of the following regarding the comparison of parallelizing
computation tasks using processes and threads is/are correct?
! Thecontextswitchandcreationoverheadofprocessesishigher
— you have to change page tables, warm up TLBs, warm up caches, create a new memory space …
” Theoverheadofexchangingdataamongdifferentcomputingtasksforthe same applications is higher in process model
# Thedemandofmemoryusageishigherwhenusingprocesses
— separate address, it’s not easy to access data from another process
— you cannot directly share data without leveraging other mechanisms
— each process needs its own address space even if most data are potentially identical
$ Thesecurityandisolationguaranteesarebetterachievedusingprocesses

Reading quizzes due next Tuesday
• Welcomenewfriends!—willdropatotalof6readingquizzesforthequarter • Attendancecountas4readingquizzes
• Weplantohaveatotalof11readingquizzes
Announcement
Office Hour links are inside Google Calendar events
• https://calendar.google.com/calendar/u/0/r?
We will make both midterm and final exams online this quarter
• AvoidtheuncertaintyofCOVID-19
• Avoidhigh-densityintheclassroom(onlysits60andwehave59fornow)during examines

Different links from lecture ones
We cannot share through any public channels so that we can better avoid Zoom bombing

Engineering

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