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C++库研究笔记——Linux 下高精度计时器

2013-07-28 13:03 465 查看
High
resolution timer in Linux

http://stackoverflow.com/questions/6749621/high-resolution-timer-in-linux

Check out 
clock_gettime
,
which is a POSIX interface to high-resolution timers.

If, having read the manpage, you're left wondering about the difference between 
CLOCK_REALTIME
and 
CLOCK_MONOTONIC
,
see Difference
between CLOCK_REALTIME and CLOCK_MONOTONIC?

See the following page for a complete example: http://www.guyrutenberg.com/2007/09/22/profiling-code-using-clock_gettime/


Linux下的高精度时间获得与定时器

在linux下通常可用的精度最高的时间接口是gettimeofday，它返回一个timeval结构，其精度为us，即10-6
秒，大多数情况这个精度已经够用了。不过有时为了更高的精度，比如纳秒级的时间精度，我们需求探索Linux为我们提供的时间调用。

测试占用的时钟周期，结果如下：

rdtsc, 22 cycles

rdtsc_serial, 294 cycles

clock, 437 cycles

gettimeofday, 53 cycles

可见，Linux下 gettimeofday 是一个折中选择

（代码来自上篇的链接）

#if defined(_MSC_VER)
	#undef inline
	#define inline __forceinline

	#pragma comment( lib, "Winmm.lib" )

	#include <windows.h>
#else
	#include <sys/time.h>
#endif

#include <math.h>
#include <stdio.h>
#include <time.h>

#include "Profiler.h"

struct rdtsc { inline void getTicks() { Profiler::Timer::getticks(); } };
struct rdtsc_serial { inline void getTicks() { Profiler::Timer::getticks_serial(); } };
struct clck { inline void getTicks() { clock(); } };

#if defined(_MSC_VER)
struct qpc { inline void getTicks() { LARGE_INTEGER li; QueryPerformanceCounter( &li ); } };
struct tgt { inline void getTicks() { timeGetTime(); } };
#else
struct gtod { inline void getTicks() { timeval t; gettimeofday( &t, NULL); } };
#endif

template< class Timer > double time( const int trials ) {
	Timer t;
	Profiler::u64 min = 1000000000000000ULL;
	for ( int run = 0; run < 5000; run++ ) {
		Profiler::Timer measure;
		{
			ScopedTimer scoped( measure );
			for ( int i = 0; i < trials; i++ )
				t.getTicks();
		}
		if ( measure.ticks < min )
			min = measure.ticks;
	}

	return Profiler::average( min, trials );
}

int main( int argc, const char *argv[] ) {
	const int trials = 1000;

	printf( "rdtsc, %.0f cycles\n", time<rdtsc>( trials ) );
	printf( "rdtsc_serial, %.0f cycles\n", time<rdtsc_serial>( trials ) );
	printf( "clock, %.0f cycles\n", time<clck>( trials ) );
#if defined(_MSC_VER)
	printf( "QueryPerformanceCounter, %.0f cycles\n", time<qpc>( trials ) );
	printf( "timeGetTime, %.0f cycles\n", time<tgt>( trials ) );
#else
	printf( "gettimeofday, %.0f cycles\n", time<gtod>( trials ) );
#endif

	return 0;
}
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