1. Chien-Chung Shen CIS/UD cshen@udel.edu
Final Exam Review
Chien-Chung Shen
CIS/UD

2. Zombie
When a parent process does not (get the chance to) do a wait() call for its children, the children will become zombie processes (marked by <defunct> in Linux or Z) when they exit /usa/cshen/public_html/361/Proj_3/zombie.c

3. Scheduling Time quantum (slice) Context switching overhead
CPU time of process
Multi-level feedback queue
zyBooks: Exercises – 3.3.4

4. Course-grained Locking
int count; int salary; // shared var’s mutex_lock A; thread 1 thread 2 lock(A); lock(A); count+=1; count+=2; salary+=50; salary+=70; unlock(A); unlock(A);
count+=1;
salary+=50;
count+=2;
salary+=70;
How to allow more threads to execute (more) different critical sections at the same time?
How to increase concurrency?
count+=1;
salary+=50;
count+=2;
salary+=70;

5. Fine-grained Locking
count+=1;
salary+=50;
count+=2;
salary+=70;
int count; int salary; // shared var’s mutex_lock thread 1 thread 2 lock(A); lock(A); count+=1; count+=2; unlock(A); unlock(A); lock(B); lock(B); salary+=50; salary+=70; unlock(B); unlock(B);
A, B;
allow more threads to execute (more) different critical sections at the same time
count+=1;
salary+=50;
count+=2;
salary+=70;

6. Semaphore Usage: Summary
Mutual exclusion: binary semaphore as mutex lock
Controlled access to a given resource consisting of a finite number of instances: counting semaphore
semaphore is initialized to the number of instances available
Synchronization:
two concurrent running threads T1 and T2 with statements S1 and S2, respectively
require S2 be executed only after S1 has completed (on one CPU)
Semaphore s = 0;
T1: S1; signal(s);
T2: wait(s); S2; S1 S2 T1 T2

7. Two threads on one CPU ➡️ either thread could run first
initially empty
int sem_wait(sem_t *s) { // P
s--;
if (s < 0) sleep; }
int sem_post(sem_t *s) { // V
s++;
if (threads waiting) wake one up; P
buffer C
empty
= 1
full
= 0
What does P care (when will P have to wait)?
P waits for buffer to become empty in order to put data into it
does it have to wait initially?
what should P do after putting data in?
notify C that buffer is full
What does C care (when will C have to wait)?
C waits for buffer to become full (filled) before getting data
what should C do after getting data?
notify P that buffer is empty
sem_wait(&empty); P
put(i);
sem_post(&full);
sem_wait(&full);
tmp = get(); C
When P looks at the buffer (initially empty), what does it care? Or when will P have to wait?
every time it has to wait for something, use a semaphore
Does it have to wait initially?
When C looks at the buffer (initially empty), what does it care? Or when will C have to wait?
sem_post(&empty);
Two threads on one CPU ➡️ either thread could run first

8. Efficiency and Concurrency
Behavior of bounded-buffer producer consumer system on a single CPU?
Essentially a stop-and-wait protocol (one put followed by one get)
many context switching between threads
How to increase its efficiency (or reduce # of context switches)?
How to increase concurrency (parallelism)?
multiple producers and multiple consumers P
buffer C

9. Single P/C + Multiple Slots
empty
= N
= ?
full
= 0
= ?

10. Rendezvous via Semaphore
/usa/cshen/361/OSTEP/Chap31/HW-Threads-RealSemaphores/r.c

11. $ ln Chapter3 Chapter3.hard $ ls –il (show attributes of files)
two names for the same file
$ ln Chapter3 Chapter3.hard
$ ls –il
(show attributes of files)

12. Get Info about Files
[cisc361:/usa/cshen/ ] echo hello > foo
[cisc361:/usa/cshen/ ] more foo
hello
[cisc361:/usa/cshen/ ] stat foo
  File: 'foo'
  Size: 6         Blocks: 3          IO Block: regular file
Device: 29h/41d Inode:       Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 4157/   cshen)   Gid: ( 4157/   cshen)
Access: :06:
Modify: :08:
Change: :08:
Birth: -
[cisc361:/usa/cshen/ ] ls -i foo
73985 foo
[cisc361:/usa/cshen/ ] 
All info of each file is stored in the inode (persistent) structure

13. Soft/Symbolic Links
$ ln –s Chapter3 Chapter3.soft
Soft link is a file itself containing “pathname” for the file that the link file is a symbolic link to
3 files types
regular file (-)
directory (d)
symbolic link (l)

14. Symbolic (Soft) Links
A symbolic link is actually a file itself, of a different type, containing the pathname of the linked-to file
d: directory
-: regular file
l: symbolic link
Possible of dangling reference

15. Fundamental Issues
Since we cannot count on simultaneous observations of global states in distributed systems, we need to find a property on which we can depend
Distributed systems are causal
the cause precedes the effect
sending of a message precedes the receipt of the message

16. time
Space-Time diagram
p1 and r4?
p3 and q3?
concurrent
causal

17. Lamport Timestamps Example
Events occurring at three processors
local logical clocks are initialized to 0 2 3 7 1 6 4 3 1 5