I'm looking for high-resolution timing code for iPhone, in order to do some performance timings. I'd like to write code like this:
HighResolutionTimer* myTimer = [[HighResolutionTimer alloc]init];
[myTimer start];
[self doSomeLengthyOperation];
NSLog( @"doSomeLengthyOperation took %f seconds", [myTimer elapsedTime] );
Look into mach_absolute_time() in the mach/mach_time.h header.
Don't use NSDate. NSDate isn't even guaranteed to not go backwards occasionally, when ntp does its thing.
(Devices can have clock drift. If the iOS device drifts fast a few seconds, then when NTP corrects this drift, you will see the clock suddenly go backwards a few seconds. Very bad for timing use. mach_time uses a counter that doesn't ever get corrected by NTP, thus can't go backwards, thus is far better for timing.)
A better option is CACurrentMediaTime()
which uses mach_absolute_time()
but converts it to a CFTimeInterval
(i.e., time in seconds as a double) for you.
Here's my answer for clock timers using both mach_absolute_time()
, based on compute method shown here, and NSDate
. The are actually the same in terms of accuracy.
double machGetClockS()
{
static bool init = 0 ;
static mach_timebase_info_data_t tbInfo ;
static double conversionFactor ;
if(!init)
{
init = 1 ;
// get the time base
mach_timebase_info( &tbInfo ) ;
conversionFactor = tbInfo.numer / (1e9*tbInfo.denom) ; // ns->s
}
return mach_absolute_time() * conversionFactor ; // seconds
}
double machGetClockDiffS()
{
static double lastTime = 0;
double currentTime = machGetClockS() ;
double diff = currentTime - lastTime ;
lastTime = currentTime ; // update for next call
return diff ; // that's your answer
}
double getClockS()
{
return [NSDate timeIntervalSinceReferenceDate] ; // NSTimeInterval is always specified in seconds
}
double getClockDiffS()
{
static double lastTime = 0 ;
double currentTime = getClockS() ;
double diff = currentTime - lastTime ;
lastTime = currentTime ; // update for next call
return diff ; // that's your answer
}
Note the resolution on both of these is really good.
IOS SIMULATOR, running frame rate counts (in milliseconds (*1000.0)) MACH_ABS_TIME / NSTimeIntervals 58.557001 / 58.552980 40.558007 / 40.562987 52.207822 / 52.200019 33.742197 / 33.742011 38.498912 / 38.504004 48.872679 / 48.868001 45.012602 / 45.011997 57.858432 / 57.865977 25.044615 / 25.038004 IPAD HARDWARE SAMPLINGS: 33.415041 / 33.416033 33.240167 / 33.239007 33.357542 / 33.357978 33.302833 / 33.302009 33.506750 / 33.509016 33.582250 / 33.582985 33.233958 / 33.232987 33.239042 / 33.237994
*If you look at the edit history of this post, you can see the danger of using float
in the place of a double
!
Use NSTimeInterval startTime = [NSDate timeIntervalSinceReferenceDate]
to get a start time, and then NSLog (@"Operation took %f seconds.", [NSDate timeIntervalSinceReferenceDate] - startTime);
at the end.
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