vc 程序效率测试 与 如何获取系统精确时间

GetTickCount返回（retrieve）从操作系统启动到现在所经过（elapsed）的毫秒数，它的返回值是DWORD。

如： DWORD time； time = GetTickCount();但是GetTickcount函数：它返回从操作系统启动到当前所经过的毫秒数，常常用来判断某个方法执行的时间，其函数原型是DWORD GetTickCount(void)，返回值以32位的双字类型DWORD存储，因此可以存储的最大值是2^32 ms约为49.71天，因此若系统运行时间超过49.71天时，这个数就会归0，MSDN中也明确的提到了:"Retrieves the number of milliseconds that have elapsed since the system was started, up to 49.7 days."。因此，如果是编写服务器端程序，此处一定要万分注意，避免引起意外的状况。特别注意：这个函数并非实时发送，而是由系统每18ms发送一次，因此其最小精度为18ms。当需要有小于18ms的精度计算时，应使用StopWatch方法进行。也就是说使用这个函数，精度不高。为了获取精确的时间，从而获得精确的效率测量可以使用QueryPerformanceCounter(&ticks)函数示例程序可以这样用：

////////////variables definition////////////
DWORD time1,time2,time3;//计算时间用
LARGE_INTEGER  frequency;//高性能计数器的频率
LARGE_INTEGER start,end;
start.QuadPart=0;
end.QuadPart=0;
frequency.QuadPart=0;
long ticks=0;
QueryPerformanceFrequency(&frequency);

///////////////////processing ///////////
#ifdef DEBUGTIME
time1 = GetTickCount();
QueryPerformanceCounter(&start);
#endif

#ifdef DEBUGTIME
time2 = GetTickCount();
QueryPerformanceCounter(&end);
ticks = (long)(end.QuadPart-start.QuadPart);
double millsecond=(double)ticks*1000.0/(double)frequency.QuadPart;
printf("noninterpolating wasted time is %ld  number %ld  ticks %lf\n",time2-time1,arrayindex,millsecond);
#endif

这是一位名叫softman11，csdn论坛里这位作者提供的c++ 类实现

#pragma once
#include<Windows.h>

///本类采用高分辨率高性能计数器实现
///在我的Intel T7500机器上，分辨率大约是279毫微秒
class StopWatch
{
public:
StopWatch(void);
~StopWatch(void);
private:
LARGE_INTEGER beginticks;
LARGE_INTEGER endticks ;
LARGE_INTEGER  frequency;//高性能计数器的频率：每秒357,9545个tick  我的INTEL T7500
public:
void Start();
void Stop();
double  GetCostMillisecond();
unsigned long long  GetFrequency();
};

#include "StdAfx.h"
#include "StopWatch.h"
#include <Windows.h>
#include<iostream>
using namespace std;

StopWatch::StopWatch(void)
{
beginticks.QuadPart=0;
endticks.QuadPart=0;
frequency.QuadPart=0;
QueryPerformanceFrequency(&frequency);
}

StopWatch::~StopWatch(void)
{
}

void StopWatch::Start()
{
//beginticks=GetTickCount();
QueryPerformanceCounter(&beginticks);

}
void StopWatch::Stop()
{
QueryPerformanceCounter(&endticks);

}
double  StopWatch::GetCostMillisecond()
{
unsigned long long cost=(unsigned long long)(endticks.QuadPart-beginticks.QuadPart);
double millsecond=(double)cost*1000.0/(double)frequency.QuadPart;
return millsecond;
}
unsigned long long StopWatch::GetFrequency()
{
return (unsigned long long)frequency.QuadPart;
}