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

#include

voidmain()

{

LARGE_INTEGERnFreq;

LARGE_INTEGERnBeginTime;

LARGE_INTEGERnEndTime;

doubletime;

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");

}

结果为

0.999982

1.000088

1.000200

等，所以Sleep的精度还是比较低的。

2. rdtsc 汇编

static inline unsigned __int64 GetCycleCount()

{

unsigned int timehi, timelo;

__asm

{

rdtsc

mov timehi, edx;

mov timelo, eax;

}

return ((unsigned __int64)timehi << 32) + (unsigned __int64)timelo;

}

rdtsc这个指令是得到CPU自启动以后的运行周期，

在586以上的CPU上有这个指令将放回的数值，

高位返回到edx，

低位放回到eax，

可以用做随机数的种子可以在一段指令的前后调用rdtsc，计算运行这段指令的时间，

不过这个指令在超线程和多核CPU上用来计算时间不是很准确了。

3. 测试实例(运行环境：VS2012，Win7，i3，2.53GHz)

#include "stdio.h"

#include "windows.h"

static inline unsigned __int64 GetCycleCount()

{

unsigned int timehi, timelo;

__asm

{

rdtsc

mov timehi, edx;

mov timelo, eax;

}

return ((unsigned __int64)timehi << 32) + (unsigned __int64)timelo;

}

int _tmain(int argc, _TCHAR* argv[])

{

//PSO参数定义

float v_in=(float)0.45;

float x_in=(float)1.32;

float pbest=(float)0.68;

float gbest=(float)0.03;

float w=(float)0.7;

float c1=(float)2.0;

float c2=(float)2.0;

float r1=(float)0.35;

float r2=(float)0.72;

float v_out=(float)0.0;

//计时程序参数定义

LARGE_INTEGER nFreq;

LARGE_INTEGER nBeginTime;

LARGE_INTEGER nEndTime;

double time;

printf("CPU Times:\n");

for(int i=0;i<10;i++)

{

QueryPerformanceFrequency(&nFreq);

QueryPerformanceCounter(&nBeginTime);

v_out=w*v_in+c1*r1*(pbest-x_in)+c2*r2*(gbest-x_in);

QueryPerformanceCounter(&nEndTime);

time=(double)1000000000.0*(nEndTime.QuadPart-nBeginTime.QuadPart)/(double)nFreq.QuadPart;

printf("%f:\t%f\n",v_out,time);

}

printf("CPU Cycles:\n");

for(int i=0;i<10;i++)

{

unsigned __int64 nBeginCycle = GetCycleCount();

v_out=w*v_in+c1*r1*(pbest-x_in)+c2*r2*(gbest-x_in);

unsigned __int64 nEndCycle = GetCycleCount();

unsigned __int64 nCycle = nEndCycle - nBeginCycle;

time=(double)nCycle/2.53;

printf("%f:\t%f\n",v_out,time);

}

return 0;

}

Result：

CPU Times:

-1.990600: 405.210849

-1.990600: 810.421699

-1.990600: 405.210849

-1.990600: 405.210849

-1.990600: 405.210849

-1.990600: 405.210849

-1.990600: 405.210849

-1.990600: 405.210849

-1.990600: 405.210849

-1.990600: 810.421699

CPU Cycles:

-1.990600: 524.901186

-1.990600: 447.430830

-1.990600: 412.648221

-1.990600: 409.486166

-1.990600: 406.324111

-1.990600: 422.134387

-1.990600: 411.067194

-1.990600: 411.067194

-1.990600: 540.711462

-1.990600: 493.280632

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