Advanced Operating Systems: Lecture 22 - Mr. Farhan Zaidi

CS703 ­ Advanced 
Operating Systems
By Mr. Farhan Zaidi


Lecture No. 
22


Mark and Sweep Collecting
Mark bit set

root
Before mark

After mark

After sweep

free

free


Mark and Sweep
Mark using depth-first traversal of the memory graph
ptr mark(ptr p) {
if (!is_ptr(p)) return;
if (markBitSet(p)) return
setMarkBit(p);
for (i=0; i < length(p); i++)

mark(p[i]);
return;
}

//
//
//
//

Sweep using lengths to find next block
ptr sweep(ptr p, ptr end) {
while (p < end) {
if markBitSet(p)
clearMarkBit();
else if (allocateBitSet(p))
free(p);
p += length(p);
}
}

do nothing if not pointer
check if already marked
set the mark bit
mark all children


What is an operating system?
Top-down view
Bottom-up view




Time multiplexing
Space multiplexing


Type of Operating Systems







Main frame operating systems
Time-sharing systems
Multiprocessor operating systems
PC operating systems
Real-time operating systems
Embedded operating systems


OS Structure




Monolithic Design
Layering
Micro-kernel

ELF Object File Format






Elf header
Program header table
.text section
.data section
.bss section


Strong and Weak Symbols


Program symbols are either strong or weak


Linker’s Symbol Rules


Rule 1. A strong symbol can only appear once.



Rule 2. A weak symbol can be overridden by a strong

symbol of the same name.



Rule 3. If there are multiple weak symbols, the linker can
pick an arbitrary one.


Creating Static Libraries
atoi.c

printf.c

Translator

Translator

atoi.o

printf.o

random.c

...

random.o

Archiver (ar)

libc.a

Translator

ar rs libc.a \
atoi.o printf.o … random.o

C standard library


The Complete Picture
m.c

a.c

Translator

Translator

m.o

a.o

libwhatever.a

Static Linker (ld)
p

libc.so

Loader/Dynamic Linker

(ld-linux.so)

p’

libm.so


Process


A unit of activity characterized by the execution of a
sequence of instructions, a current state, and an
associated set of system instructions


Private Address Spaces
0xffffffff
kernel virtual memory
(code, data, heap, stack)
0xc0000000

0x40000000

user stack
(created at runtime)

read/write segment
(.data, .bss)

0

%esp (stack pointer)

memory mapped region for
shared libraries

run-time heap
(managed by malloc)

0x08048000

memory
invisible to
user code

read-only segment
(.init, .text, .rodata)
unused

brk

loaded from the
executable file


Asynchronous Exceptions (Interrupts)


Caused by events external to the processor

Synchronous Exceptions




Traps
Faults
Aborts


Process Creation


Assign a unique process identifier



Allocate space for the process



Initialize process control block


Unix SVR4 Processes


Process Control Block






Process state: e.g., running, ready, waiting, halted
Priority
Scheduling-related information
Event


fork: Creating new processes


int fork(void)


exit: Destroying Process


void exit(int status)


Zombies


Idea



Reaping

exec: Running new programs


int execl(char *path, char *arg0, char *arg1, …,0)


Safe and Unsafe Trajectories
Thread 2

T2

Safe trajectory

S2
critical
section
wrt cnt

Unsafe
trajectory

Unsafe region

U2
L2
H2
H1

L1

U1

S1

critical section wrt cnt

T1

Thread 1


Semaphores


semaphore = a synchronization primitive
– higher level than locks
– invented by Dijkstra in 1968, as part of the THE OS