CH6 Synchronization Tools

6.0 Cooperating Process

• Cooperating process (合作行程)
  • A process is cooperating if it can affect or be affected by the other processes executing in the system
  • Any process that shares data with other processes is a cooperating process
  • How to share data?
    • Message passing
    • Shared memory → 同時間的存取會造成資料不一致

6.1 Background

合作行程可用 Producer-consumer 問題來闡述，Producer負責產生資訊，並由consumer使用。

假如我們要設計一個buffer在shared memory中讓生產者和消費者共享資料…

 #define BUFFER SIZE 10
 typedef struct {
	 . . .
 } item;
 
 // circular array
 item buffer[BUFFER SIZE];
 int in = 0; // in 指向buffer的下一個空位
 int out = 0; // out 指向buffer第一個填滿的位置
 int count = 0; // array有的item數量
 item next_produced, next_consumed;

// Producer process
while (true) 
{
	 /* produce an item in next produced */
	 while (count == BUFFER SIZE)
			 ; /* do nothing */
	 buffer[in] = next_produced;
	 in = (in + 1) % BUFFER SIZE;
	 count++;
}

 // Consumer process
 while (true) 
 {
	 while (count == 0)
			 ; /* do nothing */
	 next_consumed = buffer[out];
	 out = (out + 1) % BUFFER SIZE;
	 count--;
	 /* consume the item in next consumed */
 }

count 可能因為生產者和消費者並行執行count++和count- -而不會是正確的值

count++ 在底層的機器指令可能為…

register1 = count
register1 = register1 +1
count = register1

count- -

 register2 = count
 register2 = register2 −1
 count = register2