Lecture 6: Synchronization

Question 1

define race condition

Answer 1

several process access and manipulate the same data at the same time so that the outcome of execution is depends on the order of process executioln

Question 2

what is critical section

Answer 2

• process may be changing common variables, updating table, writing file, …
• critical section problem design protocol to solve race condition
• each process need to ask permission to enter critical section in entry section, follow by exit section and remaining section

Question 3

define mutual exclusive

Answer 3

if a process,P1 is executing in its critical section, no other process can enter its critical section

Question 4

define bound waiting

Answer 4

bound must exist when process are allowed to enter their critical section
- after request is made
- before request is granted

Question 5

Peterson’s Solution

Answer 5

• used to restrict two process to alternate execute between critical section and remaining section
• assume LOAD and STORE cannot be interrupt
• sharing 2 variables: int turn and boolean flag[2]

Question 6

int turn variables

Answer 6

determine whose turn to enter the critical section

Question 7

boolean flag

Answer 7

determine if a process is ready to enter the critical section

Question 8

critical section problem’s solution requirement

Answer 8

• bound waiting requirement
• progress requirement
• mutual exclusive

Question 9

other solution for critical section problem

Answer 9

• lock
• test memory word and set value: test_and_set()
• swap content of two memory word: compare_and_swap()

Question 10

how Mutex Lock work

Answer 10

• process acquire a lock before entering critical section
• then release the lock after exit the critical section

Question 11

what is semaphore S

Answer 11

an integer variable that can only accessed through wait() and signal()

Question 12

counting semaphore

Answer 12

• initialized to number of resources available
• process perform a wait() in order to use the resource
• process perform a signal when release the resource
• all resources are being used when semaphore count = 0
• process will be blocked until semaphore count >0

Question 13

3 classical problems of synchronization

Answer 13

• bounded-buffer problem
• reader and writer problem
• dining’s philosophers problem

Question 14

problem with dining philosophers

Answer 14

• possibly create deadlock
• all philosophers hungry at the same time

Question 15

how to prevent deadlock in dining philosophers problem

Answer 15

• allow at most n-1 philosophers
• allow pick up when both chopstick are available
• asymmetric solution: odd number pick left first and even number pick right first

Question 16

define progress

Answer 16

if no process in critical section, the selection of the next process cannot be postponed indefinitely

Question 17

disadvantages of reader and writer problem

Answer 17

• allow multiple reader read at the same time
• only one writer can access the data at one time