clock_gettime性能不一致

Question

我想花時間使用一些C++代碼，但是我得到的結果很奇怪。我寫了這個測試程序來試圖隔離發生了什麼。任何人都可以解釋結果嗎？這是如果這是有關在Ubuntu 11.04和EC2中高CPU實例運行

#include <iostream> 
#include <time.h> 
using namespace std; 

int main() 
{ 
    timespec startTime, currentTime; 
    long elapsed; 

    for (int i=0; i<5; i++) { 
     clock_gettime(CLOCK_REALTIME, &startTime); 
     sleep(1); 
     clock_gettime(CLOCK_REALTIME, &currentTime); 
     elapsed = currentTime.tv_nsec - startTime.tv_nsec; 
     cout << elapsed << " nanoseconds elapsed" << endl; 
     cout << "1000000000 expected" << endl; 
    } 
    return 0; 
} 

輸出：

109044 nanoseconds elapsed 
1000000000 expected 
133713 nanoseconds elapsed 
1000000000 expected 
197287 nanoseconds elapsed 
1000000000 expected 
143396 nanoseconds elapsed 
1000000000 expected 
111871 nanoseconds elapsed 
1000000000 expected 

Answer 1

你也必須考慮到，目前的時間可能已經結轉到下一秒鐘。納秒字段只會告訴你在當前時間內你已經走了多遠。

elapsed = (currentTime.tv_sec - startTime.tv_sec) * 1000000000 
       + currentTime.tv_nsec - startTime.tv_nsec;