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How to get timings as fine-grained as one nanosecond or better

, 2 Dec 2003 146.6K 40
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How to get timings as fine-grained as one nanosecond or better

Introduction

Everyone has used good old time(NULL) to get timings accurate to the second. Some of you may have used GetSystemTime() to get sub-second timings. The really clever have found QueryPerformanceFrequency and QueryPerformanceCounter, which give timings accurate to a millisecond or better (for the record, I'm not among the really clever - I found out about those two by reading the Python documentation... Thanks, Guido!).

But if you really want accurate timings, this code will give you timings accurate to the machine cycle, which on a 1 ghz machine is one nanosecond. On a 2 ghz machine, it's 1/2 nanosecond. Old 100 mhz machines will only get 10-nanosecond timings. You get the idea.

It requires a tiny bit of assembler, but it's worth it:

__int64 GetMachineCycleCount()
{      
   __int64 cycles;
   _asm rdtsc; // won't work on 486 or below - only pentium or above
   _asm lea ebx,cycles;
   _asm mov [ebx],eax;
   _asm mov [ebx+4],edx;
   return cycles;
}

This code will work on Win9X or NT/2K and probably XP. Actually, it would even work in Linux if you can figure out how to get GCC to emit inline assembler!

Of course, the time comes out in cycles, not seconds, and oddly there seems to be no API to get the machine's speed. You can either "calibrate" the results (by getting the count, sleeping for (say) one second, getting the count again, and doing some trivial math), or just use cycles directly and don't worry about seconds (it's great for comparing one algorithm to another, or finding the slow parts of your program)

One warning, however: certain machines, notably laptops, can slow down their processor speed when nothing important seems to be happening. Since the calibration sleep is exactly one of those times, you can get some seriously wrong results from the calibration.

License

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About the Author

Mark VanTassel
Web Developer
United States United States
No Biography provided

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Comments and Discussions

 
GeneralToo bad you didn't research more on this Pin
amir.tet30-Aug-09 5:45
memberamir.tet30-Aug-09 5:45 
QuestionInconsistancy with Vista. Pin
Jose Praveen22-Oct-08 17:02
memberJose Praveen22-Oct-08 17:02 
AnswerRe: Inconsistancy with Vista. Pin
joshua01376-Jan-09 18:18
memberjoshua01376-Jan-09 18:18 
GeneralC# Pin
screig13-Jul-06 5:28
memberscreig13-Jul-06 5:28 
GeneralPlease verify this ...... Pin
Anonymous25-Mar-05 1:41
sussAnonymous25-Mar-05 1:41 
GeneralRe: Please verify this ...... Pin
Anonymous25-Mar-05 3:36
sussAnonymous25-Mar-05 3:36 
Questiontimer out of this? Pin
nikoladsp30-May-04 23:57
membernikoladsp30-May-04 23:57 
GeneralWhat's the point Pin
Anonymous12-Dec-03 1:10
sussAnonymous12-Dec-03 1:10 
GeneralRe: What's the point Pin
nastyimp1316-Nov-05 10:16
membernastyimp1316-Nov-05 10:16 
GeneralRe: What's the point Pin
Robert Bielik9-Jun-06 2:22
memberRobert Bielik9-Jun-06 2:22 
GeneralQueryPerformanceFrequency Pin
parisitic10-Dec-03 15:58
memberparisitic10-Dec-03 15:58 
AnswerRe: QueryPerformanceFrequency Pin
admiralh216-Mar-07 7:57
memberadmiralh216-Mar-07 7:57 
Generalclobbering ebx Pin
LBMT10-Dec-03 14:40
memberLBMT10-Dec-03 14:40 
GeneralRe: clobbering ebx Pin
dCp30312-Dec-03 5:10
memberdCp30312-Dec-03 5:10 
GeneralRe: clobbering ebx Pin
LBMT12-Dec-03 5:42
memberLBMT12-Dec-03 5:42 
QuestionHow about other Intel CPU's Pin
Mad_C9-Dec-03 20:26
memberMad_C9-Dec-03 20:26 
GeneralRDTSC for Watcom C/C++ Pin
c2j29-Dec-03 20:25
memberc2j29-Dec-03 20:25 
QuestionWill HT processor work? Pin
Vincent Leong779-Dec-03 19:04
memberVincent Leong779-Dec-03 19:04 
AnswerRe: Will HT processor work? Pin
c2j29-Dec-03 20:31
memberc2j29-Dec-03 20:31 
AnswerRe: Will HT processor work? Pin
tmangan10-Dec-03 1:58
membertmangan10-Dec-03 1:58 
GeneralSomewhat disappointed Pin
ReorX4-Dec-03 9:51
memberReorX4-Dec-03 9:51 
GeneralRe: Somewhat disappointed Pin
noshbar8-Dec-03 23:45
membernoshbar8-Dec-03 23:45 
GeneralCode to query CPU frequency Pin
Zoltan Csizmadia4-Dec-03 8:54
memberZoltan Csizmadia4-Dec-03 8:54 
GeneralRe: Nicer format Pin
WREY4-Dec-03 9:56
memberWREY4-Dec-03 9:56 
GeneralRe: Nicer format Pin
thought4-Dec-03 21:45
memberthought4-Dec-03 21:45 

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