1. Chapter 6: Process Synchronization

2. Process Synchronization
Background
The Critical-Section Problem
Peterson’s Solution
Synchronization Hardware
Semaphores
Classic Problems of Synchronization
Monitors
Synchronization Examples
Atomic Transactions

3. Objectives
To introduce the critical-section problem, whose solutions can be used to ensure the consistency of shared data
To present both software and hardware solutions of the critical-section problem
To introduce the concept of an atomic transaction and describe mechanisms to ensure atomicity

4. Background
Concurrent access to shared data may result in data inconsistency
Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes「意義上等價於某個執行順序即可」

5. Background - Example
Suppose that we wanted to provide a solution to the consumer-producer problem that fills all the buffers.
We can do so by having an integer count that keeps track of the number of full buffers.
Initially, the count is set to 0.
It is incremented by the producer after it produces a new buffer and is decremented by the consumer after it consumes a buffer.

6. Producer
while (true) { /* produce an item and put in nextProduced */ while (count == BUFFER_SIZE) ; // do nothing buffer [in] = nextProduced; in = (in + 1) % BUFFER_SIZE; count++; }

7. Consumer
while (true) { while (count == 0) ; // do nothing nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; count--; /*consume the item in nextConsumed*/ }

8. Race condition Producer (Thread 1) Consumer (Thread 2)
while (true) { /* produce an item and put in nextProduced */ while (count == BUFFER_SIZE) ; // do nothing buffer [in] = nextProduced; in = (in + 1) % BUFFER_SIZE; count++; }
while (true) { while (count == 0) ; // do nothing nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; count--; /*consume the item in nextConsumed*/ }

9. Race Condition
count++ could be implemented as register1 = count register1 = register count = register1
count-- could be implemented as register2 = count register2 = register count = register2

10. Race Condition Consider this execution interleaving with “count = 5” initially
Producer
Consumer
register1 = count {register1 = 5} register1 = register1 + 1 {register1 = 6} count = register1 {count = 6 } R
register2 = count {register2 = 5} register2 = register2 - 1 {register2 = 4} count = register2 {count = 4} R w w

11. Race Condition Consider this execution interleaving with “count = 5” initially
Producer
Consumer
register1 = count {register1 = 5} register1 = register1 + 1 {register1 = 6} count = register1 {count = 6 } R
register2 = count {register2 = 6} register2 = register2 - 1 {register2 = 5} count = register2 {count = 5} w R w

12. 深入思考 使用Count實現 使用in, out實現 二個process都會對Count進行讀寫 節省記憶體的使用量
只有一個process會對in進行寫入，另一個process對in只會讀取
只有一個process會對out進行寫入，另一個process對out只會讀取
增加平行度

13. 有很多方法解決race condition，critical section是其中一種方式！！

14. Solution to Critical-Section Problem
1. Mutual Exclusion - If a process is executing in its critical section, then no other processes can be executing in their critical sections 2. Progress - If no process is executing in its critical section and there exist some processes that wish to enter their critical section, then the one of the processes will enter the critical section 3. Bounded Waiting - A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted

15. Solution to Critical-Section Problem
Mutual Exclusion
“計算結果”的正確性
Progress
有效率的使用資源
Bounded Waiting
在某個程度上資源的分配是公平的

16. Peterson’s Solution Two process solution
Assume that the LOAD and STORE instructions are atomic; that is, cannot be interrupted.
The two processes share two variables:
int turn;
Boolean flag[2]
The variable turn indicates whose turn it is to enter the critical section.
The flag array is used to indicate if a process is ready to enter the critical section. flag[i] = true implies that process Pi is ready!

17. Peterson’s Solution
純粹軟體？

18. Peterson’s Solution
純粹軟體？
假設CPU對memory的寫入及讀出動作是「atomic operation」

19. Algorithm for Process Pi
while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }

20. 並且flag[i]及flag[j]已經給了正確的值
Mutual Exclusion
while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }
while (true) { flag[j] = TRUE; turn = i; while ( flag[i] && turn == i); /*CRITICAL SECTION*/ flag[j] = FALSE; /*REMAINDER SECTION*/ }
turn = i或j；
並且flag[i]及flag[j]已經給了正確的值

21. turn=j；但flag[j]=false
progress
while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }
while (true) { flag[j] = TRUE; turn = i; while ( flag[j] && turn == i); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }
turn=j；但flag[j]=false

22. Bounded Waiting 展開loop (loop unrolling)
while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }
flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/
展開loop
(loop unrolling)

23. Bounded Waiting
flag[j] = TRUE; turn = i; while ( flag[i] && turn == i); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/
flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/

24. Algorithm for Process Pi
while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }

25. Algorithm for Process Pi (?????)
while (true) { turn = j; flag[i] = TRUE; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }

26. Algorithm for Process Pi
Algorithm for Process Pj
while (true) { turn = j; flag[i] = TRUE; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }
while (true) { turn = i; flag[j] = TRUE; while ( flag[i] && turn == i); /*CRITICAL SECTION*/ flag[j] = FALSE; /*REMAINDER SECTION*/ }

27. Synchronization Hardware
Many systems provide hardware support for critical section code
Uniprocessors – could disable interrupts
Currently running code would execute without preemption
Generally too inefficient on multiprocessor systems
Modern machines provide special atomic hardware instructions
test memory word and set value
swap contents of two memory words

28. TestAndndSet Instruction
Definition:
boolean TestAndSet (boolean *target) {
boolean rv = *target;
*target = TRUE;
return rv: }

29. Solution using TestAndSet
Shared boolean variable lock., initialized to false.
while (true) {
　while ( TestAndSet (&lock ))
　　; // do nothing
　　// critical section
　　lock = FALSE;
　　// remainder section }

30. Swap Instruction Definition: void Swap (boolean *a, boolean *b) {
　boolean temp = *a;
　*a = *b;
　*b = temp: }

31. Solution using Swap
Shared Boolean variable lock initialized to FALSE; Each process has a local Boolean variable key.
while (true) {
　　key = TRUE;
　　while ( key == TRUE) {
　　　Swap (&lock, &key ); }
　　　//critical section
　　　lock = FALSE;
　　　//remainder section

32. Semaphore “一個較為抽象化的同步運算子”

33. Semaphore Synchronization tool that does not require busy waiting
Semaphore S – integer variable
Two standard operations modify S: wait() and signal()

34. Semaphore signal (S) { S++; } The definition of wait(): wait (S) {
while (S <= 0)
; // no-op
S--; }
The definition of signal():
signal (S) {
S++; }

35. Semaphore The definition of semaphore S: S=0 S>0 S<0
S: 有多少人可以進入到critical section
S=0
沒有人
S>0
有S個人可以進去
S<0
有S個人已經進去（需要等他們都離開）
有|S|的人在等待

36. Semaphore as General Synchronization Tool
Counting semaphore – integer value can range over an unrestricted domain
Binary semaphore – integer value can range only between 0 and 1; can be simpler to implement
Also known as mutex locks
Provides mutual exclusion
Semaphore S; // initialized to 1
wait (S);
　　//Critical Section
signal (S);

37. Semaphore Implementation
wait (S) {
while (S <= 0)
; // no-op
S--; }
signal (S) {
S++; }
Busy waiting wastes CPU time!
This type of semaphore also called “spinlock”
(coding in assembly)

38. Semaphore Implementation with no Busy waiting
With each semaphore there is an associated waiting queue. Each entry in a waiting queue has two data items:
value (of type integer)
pointer to next record in the list
Two operations:
block – place the process invoking the operation on the appropriate waiting queue.
wakeup – remove one of processes in the waiting queue and place it in the ready queue.

39. Semaphore Implementation with no Busy waiting
wait (S){ value--; if (value < 0) { add this process to waiting queue block(); //krl_fn_sleep() } }
Signal (S) { value++; if (value <= 0) { remove a process P from the waiting queue wakeup(P); } }
(coding in assembly)