計算時鐘週期給出了截然不同的結果

Question

我的代碼計算時鐘週期來創建線程

# include <Windows.h> 
# include <stdio.h> 
# include <conio.h> 

# define RUN 20000 

DWORD WINAPI Calc(LPVOID Param){} 

int main(int argc, char* agrv[]) 
{ 
    ULONG64 Sum = 0; 

    for (int i = 0; i < RUN; i++) 
    { 
     ULONG64 ret = 0; 
     DWORD ThreadId; 
     HANDLE ThreadHandle; 

     /* create the thread */ 
     ThreadHandle = CreateThread(
      NULL,   /* default security attributes */ 
      0,    /* default stack size */ 
      Calc,   /* thread function */ 
      NULL,   /* parameter to thread function */ 
      0,    /* default creation flags */ 
      &ThreadId);  /* returns the thread identifier */ 

     QueryThreadCycleTime(ThreadHandle, &ret); 

     WaitForSingleObject(ThreadHandle, INFINITE); 

     CloseHandle(ThreadHandle); 

     Sum += ret; 
    } 

    printf_s("The Average no of cycles in %d runs is %lu\n", RUN, (DWORD)(Sum/RUN)); 

    _getch(); 
    return 0; 
} 

這個代碼的結果是圓我的微薄的筆記本電腦約1000個時鐘週期。但是，如果在WaitForSingleObject函數之後調用QueryThreadCycleTime函數，則結果會非常不同，大小約爲200,000。我環顧四周，但沒有真正找到解釋。這種行爲的原因是什麼？

Answer 1

不同之處在於您是否等待線程完成執行。很明顯，如果你等待，那麼這樣做將允許線程使用更多的時鐘週期。

請注意，您不計時創建線程的過程。您正在計時執行線程過程。讓我清楚，由QueryThreadCycleTime返回的值是執行線程所消耗的週期數，而不是執行CreateThread所花費的週期數，或者實際上是從調用CreateThread開始執行的線程所花費的掛鐘時間。

而你這樣做是在一個不確定的點。例如，在您的代碼中，QueryThreadCycleTime有時會返回0，因爲該線程在主線程調用QueryThreadCycleTime時甚至沒有開始執行。

如果您想要創建線程時間，則需要多長時間才能返回CreateThread。或者更好的方法是，測量在撥打CreateThread之間經過的掛鐘時間，並開始執行線程。

例如，代碼可能是這樣的：

# include <Windows.h> 
# include <stdio.h> 
# include <conio.h> 

# define RUN 100000 

DWORD WINAPI Calc(LPVOID Param){ 
    QueryPerformanceCounter((LARGE_INTEGER*)Param); 
} 

int main(int argc, char* agrv[]) 
{ 
    ULONG64 Sum = 0; 

    for (int i = 0; i < RUN; i++) 
    { 
     LARGE_INTEGER PerformanceCountBeforeCreateThread, PerformanceCountWhenThreadStartsExecuting; 
     DWORD ThreadId; 
     HANDLE ThreadHandle; 

     /* create the thread */ 
     QueryPerformanceCounter(&PerformanceCountBeforeCreateThread); 
     ThreadHandle = CreateThread(NULL, 0, Calc, 
      &PerformanceCountWhenThreadStartsExecuting, 0, &ThreadId);  
     WaitForSingleObject(ThreadHandle, INFINITE); 
     CloseHandle(ThreadHandle); 

     Sum += PerformanceCountWhenThreadStartsExecuting.QuadPart - PerformanceCountBeforeCreateThread.QuadPart; 
    } 

    printf_s("The Average no of counts in %d runs is %lu\n", RUN, (DWORD)(Sum/RUN)); 

    LARGE_INTEGER Frequency; 
    QueryPerformanceFrequency(&Frequency); 
    printf_s("Frequency %lu\n", Frequency.QuadPart); 

    _getch(); 
    return 0; 
}