gcc high resolution timer? 2

[fimion]

okay, working on porting some code from windows to linux and i've gotten stuck. It's a game server and we have a timer class, which needs to be a high resolution timer (hence why i can't just use time.h)

currently, here's how the game creator descibes the Timer class:

When POG starts up, it first calls Timer::init_frequency() to set ms_frequency to however many "hardware timer ticks" there are per second. This frequency is different on every computer, but it never changes while the computer is running.

QueryPerformanceCounter() reads the current value of the hardware timer. Suppose you call this function once and put the result value into a variable. Then you call it again at a later time, put the result into another variable. Then you subtract the first variable's value from the second, and you'll get however many hardware ticks have passed. Divide that by the frequency and you get however many seconds have passed. m_lengthOneTick holds however many hardware ticks there are in the Timer's tick.

I think those functions ultimately wind up using the x86 processor's RDTSC instruction to read the processor's high resolution hardware timer. You'll have to find whatever Linux functions do the same thing.

and here's how the class is layed out:

class Timer
{
public:
	Timer(float ticksPerSecond);
	bool TickHasPassed();
	bool PeekTickHasPassed();
	bool PeekTickHasAlmostPassed();
	bool PeekLessThanQuarterTickHasPassed();
	bool PeekHalfTickHasPassed();
	bool PeekTickPlusAQuarterHasPassed();
	bool PeekTwoTicksHavePassed();
	void MarkTickHasPassed();
	void Reset();
	void HalfSleep();
	float count_elapsed_ticks();

	void AdjustOnFly(float ticksPerSecond);

	UINT GetSecondsLeftInTick();
	UINT GetSecondsInTick();

	float get_seconds_left_in_tick_float();

	static UINT GetRandom(UINT modNumber);

	static LARGE_INTEGER ms_frequency;
	static LARGE_INTEGER ms_freq_div_30;
	static bool ms_frequency_set;

	static __int64 get_time_code ();

	static void init_frequency ()
	{
		QueryPerformanceFrequency(&ms_frequency);
		ms_freq_div_30.QuadPart = ms_frequency.QuadPart / 30;
		ms_frequency_set = true;
	}

private:
	LARGE_INTEGER m_startCounter, m_counter;
	__int64 m_lengthOneTick;
};

if someone could point me in the right direction of a high res timer for gcc so i can redo the class, it would be most appreciated.

 

[cpjust]

If I'm understanding the description of the RDTSC instruction properly; as long as Linux doesn't set the TSD flag in the CR4 register, you can use the RDTSC instruction in your program.

How good are you with Assembly code?

 

[fimion]

absolutely no experience what so ever.

 

[cpjust]

Maybe you don't even need to know assembly. I tried out the rdtsc() function on this site:

http://www.linux.com/howtos/IO-Port-Programming-4.shtml

and it seemed to work, although I had to remove the 'extern' keyword.

__inline__ unsigned long long int rdtsc()
{
    unsigned long long int x;
    __asm__ volatile (".byte 0x0f, 0x31" : "=A" (x));
    return x;
}

Don't ask me what that inline assembly line means, because I don't have a clue...

 

[Salem]

Newer gcc compilers understand the instruction itself, so
[tt]__asm__ volatile ("rdtsc" : "=A" (x));
[/tt]

--

 

[cpjust]

I still don't know what the : "=A" (x) part means?

 

[fimion]

so in order to find the frequency of the processor, should i call gettimeofday(), then call rdtsc(), then gettimeofday() again, subtract the latter from the first, then divide 1 by that number?

 

[Salem]

> I still don't know what the : "=A" (x) part means?

http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html

> so in order to find the frequency of the processor
That I don't know - I mis-read that part of the question. I thought you were just after the counter, not the frequency.


--

 

[cpjust]

I thought you'd have to do something like this:

long long int first = rdtsc();
sleep( 1 );  // Sleep 1 second.
long long int second = rdtsc();
long long int frequency = second / first;

 

[fimion]

Thank you! i knew it involved something of doing something once and the other before and after, but i wasn't sure what.