iPhone的高分辨率计时器？

Question

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] );

Answer 1

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.)

Answer 2

更好的选择是CACurrentMediaTime() ，它使用mach_absolute_time()但会为您将其转换为CFTimeInterval （即，以秒为单位的时间为两倍）。

Answer 3

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.

Mach version

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
}

NSTimeInterval version

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
}

Results:

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 !}

Answer 4

Use NSTimeInterval startTime = [NSDate timeIntervalSinceReferenceDate] to get a start time, and then NSLog (@"Operation took %f seconds.", [NSDate timeIntervalSinceReferenceDate] - startTime); at the end.