C++统计精确时间

QueryPerformanceFrequency用法

精确获取时间：

QueryPerformanceFrequency() - 基本介绍

类型：Win32API

原型：BOOL QueryPerformanceFrequency(LARGE_INTEGER *lpFrequency);

作用：返回硬件支持的高精度计数器的频率。

返回值：非零，硬件支持高精度计数器；零，硬件不支持，读取失败。

QueryPerformanceFrequency() - 技术特点

供WIN9X使用的高精度定时器：QueryPerformanceFrequency()和QueryPerformanceCounter()，要求计算机从硬件上支持高精度定时器。需包含windows.h头文件。

函数的原形是：

BOOL QueryPerformanceFrequency(LARGE_INTEGER *lpFrequency);

BOOL QueryPerformanceCounter (LARGE_INTEGER *lpCount);

数据类型LARGEINTEGER既可以是一个作为8字节长的整数，也可以是作为两个4字节长的整数的联合结构，其具体用法根据编译器是否支持64位而定。该类型的定义如下：

typeef union _ LARGE_INTEGER

{

struct

{

DWORD LowPart;

LONG HighPart;

};

LONGLONG QuadPart;

} LARGE_INTEGER;

在定时前应该先调用QueryPerformanceFrequency()函数获得机器内部计时器的时钟频率。接着在需要严格计时的事件发生前和发生之后分别调用QueryPerformanceCounter()，利用两次获得的计数之差和时钟频率，就可以计算出事件经历的精确时间。

测试Sleep的精确时间：

#include <stdio.h>

#include <windows.h>

void main()

{

     LARGE_INTEGER nFreq;

     LARGE_INTEGER nBeginTime;

     LARGE_INTEGER nEndTime;

     double time;

 

     QueryPerformanceFrequency(&nFreq);

     QueryPerformanceCounter(&nBeginTime); 

 

     Sleep(1000);

 

     QueryPerformanceCounter(&nEndTime);

     time=(double)(nEndTime.QuadPart-nBeginTime.QuadPart)/(double)nFreq.QuadPart;

 

     printf("%f\n",time);

     Sleep(1000);

system("Pause");

}

以上转自： http://www.cppblog.com/deane/articles/113151.html

再给出我写的一个Fibonacci的例子：

#include <iostream>
#include <cmath>
#include <ctime>
#include <windows.h>
#include<iomanip>
using namespace std;

unsigned long Fib1(unsigned long n)
{
    return (n == 1 || n == 2) ? 1 : Fib1(n - 1) + Fib1(n - 2);
}

unsigned long Fib2(unsigned long n)
{
    if (n == 1 || n == 2)
    {
        return 1;
    }
    unsigned long m1 = 1, m2 = 1;
    for (unsigned long i = 3; i <= n; i++)
    {
        m2 = m1 + m2;
        m1 = m2 - m1;
    }

    return m2;
}

int main()
{
    LARGE_INTEGER nFreq;

    LARGE_INTEGER nBeginTime;

    LARGE_INTEGER nEndTime;

    double time1 = 0, time2 = 0;
    unsigned long result1 = 0;
    unsigned long result2 = 0;
    unsigned long fib_num = 45;
    cout.precision(10);
    cout.setf(cout.showpoint); //设置为始终输出小数点后的数字，就是说 a = 3，它也输出 3.00000 这
    QueryPerformanceFrequency(&nFreq);

    QueryPerformanceCounter(&nBeginTime);

    result1 = Fib1(fib_num);

    QueryPerformanceCounter(&nEndTime);

    time1=(double)(nEndTime.QuadPart-nBeginTime.QuadPart)/(double)nFreq.QuadPart;

    cout<<"Fib1: "<< fixed<<setprecision(8)<<setiosflags(ios::showpoint)<<time1<<endl;


    QueryPerformanceFrequency(&nFreq);

    QueryPerformanceCounter(&nBeginTime);

    result2 = Fib2(fib_num);

    QueryPerformanceCounter(&nEndTime);

    time2=(double)(nEndTime.QuadPart-nBeginTime.QuadPart)/(double)nFreq.QuadPart;

    cout<<"Fib2: "<< fixed<<setprecision(20)<<setiosflags(ios::showpoint)<<time2<<endl;

    cout<<"Fib2 cost "<<time2*(unsigned long)100/time1<<"% time of Fib1."<<endl;
    //cout << "Hello world!" << endl;
    return 0;
}