Synchronization mechanisms

Question 1

what is the critical section problem

Answer 1

each process has a critical section that is mutually exclusive - if one process is in its critical section, no other process may be in its critical section

how to design a protocol to solve this

Question 2

Name the three requirements for solutions to critical section problem

Answer 2

mutual exclusion
progress
bounded waiting

Question 3

what is meant by mutual exclusion

Answer 3

if process a is in its critical section, no other process can be in its critical section

Question 4

what is meant by progress (requirement for solution to crit section problem)

Answer 4

any process wanting to enter its critical section should be allowed to compete to enter and we should be able to make the decision in a reasonable amount of time

Question 5

what is meant by bounded waiting

Answer 5

there exists a limit on the number of times that other processses are allowed to enter their critical sections after a process has made a request to enter its critical section

basically if a process requests to enter its crit section, there should be a limit to the number of other processes that will exectute their crit sections before it - i.e. should be granted access to its critical section in a reasonable amount of time

Question 6

Name and explain some hardware solutions to synchronization

Answer 6

test-and-set() - combines the actions of testing and setting a variable into a single machine instruction that cannot be interrupted
compare-and-swap() - swaps the value of a variable only if equal to the expected value

do not satisfy bounded waiting

Question 7

Name some software solutions

Answer 7

mutex lock

semaphores

Question 8

Different types of semaphores

Answer 8

binary

counting

Question 9

what are monitors

Answer 9

high level abstraction that provides a convenient and effective mechanism for process synchronization
- class in which all data is private and onlt one method within any given monitor is active at a time

monitor methods may only access the shared data within the monitor and nay data passed to them as parameters.. i.e. they cannot access any data external to the monitor

Question 10

what are condition variables

Answer 10

condition x;
x.wait() - suspend process until x.signal() is called

there is not much detail about this?

Question 11

Signaling pattern with semaphores

Answer 11

See physical notes

A1 before B1

Thread A
a1()
sem.signal()

Thread B
sem.wait()
b1

Question 12

Rendezvous pattern

Answer 12

See physical notes

a1 before b2 and b1 before a2

Thread a

a1()
aSem.signal()
bSem.wait()
a2()

Thread b

b1()
bSem.signal()
aSem.wait()
b2()

Question 13

Barriers

Answer 13

Lecture video - physical notes