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rdtsc help

Started by simon10k May 17, 2006 at 6:04 AM 6 replies 4.1k views

Original Post

May 17, 2006 at 6:04 AM

I was just wondering if this is the correct way to use the rdtsc


unsigned __int64 get_cpu_tick() 
{	
	DWORD dwLow, dwHigh;

	__asm 
	{
		rdtsc
		mov dwLow, eax
		mov dwHigh, edx
	}

	return ((unsigned __int64) dwHigh << 32 | dwLow); 
}

or am I missing something? Thanks. ~edit~ I found another function, unsigned __int64 __rdtsc(void); Which does what I need, another question, does QueryPerformanceFrequency() return the frequency of the processsor?

Kitt3n

May 17, 2006 at 6:54 AM

>Which does what I need, another question, does QueryPerformanceFrequency()
>return the frequency of the processsor?
It returns the number of counts / second which the high-frequency timer has.

Beware that using rdtsc on multi-core cpu's _will_ give you problems
(actually qpc/qpf will also, if you don't install a special win-xp patch).

nmi

May 17, 2006 at 7:06 AM

Quote:
Original post by Kitt3n
Beware that using rdtsc on multi-core cpu's _will_ give you problems
(actually qpc/qpf will also, if you don't install a special win-xp patch).

Maybe you can try RDTSCP, which will also return another value in ecx to identify the process (at least if your processor supports it, mine does not).

If you want to time very short functions (i.e. to get the cycle count), you may also want to put a serializing instruction in front of RDTSC, i.e. CPUID is one.

To time your code using RDTSC, you can set the thread affinity to keep it on a single cpu.

Ravuya

296

Ravuya

May 17, 2006 at 9:11 AM

Be forewarned: From experimentation RDTSC doesn't appear to work right on AMD64s or dual-core machines. I haven't bothered to research a solution yet.

simon10k

Author

simon10k

May 17, 2006 at 3:09 PM

Another thing, can I convert the rdtsc into ms?

nmi

May 17, 2006 at 3:19 PM

Quote:
Original post by simon10k
Another thing, can I convert the rdtsc into ms?

If you measured a reference value for your ticks, you can divide the number of ticks by the tick frequency.

Rockoon1

May 17, 2006 at 3:25 PM

rdtsc also suffers from cpu's which throttle down (Pentium M?) to save energy

Kitt3n

May 17, 2006 at 3:31 PM

>rdtsc also suffers from cpu's which throttle down (Pentium M?) to save energy
Same thing goes for qpc/qpf

>Maybe you can try RDTSCP, which will also return another value in ecx to
>identify the process (at least if your processor supports it, mine does not).
www.intel.com mentions a way to determine on which cpu code was just executed (look for APIC).

>To time your code using RDTSC, you can set the thread affinity to keep it
>on a single cpu.
Easiest way, yes - but using the APIC, you can actually make rdtsc multicore
safe. It involves running a thread on all cpu's in realtime priority and
letting them simultaneously get an initial/reference-timestamp.

From then on each time you get a timestamp, check which cpu it ran on
and subtract the initial timestamp.

This still will give problems with throttle-down though...

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