High-resolution timer for iPhone?

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] );

Donley

Why not just use getrusage()? Anything that works in C will work in Objective-C. – Ballroom

Because there's a perfectly good Cocoa method for doing the same thing. – Angi

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.)

Authoritarian

NSDate just returns a double that counts the seconds since Jan 1 2001. For stopwatch timing, a negative time just won't occur, so I see no need for the overhead of bringing mach_absolute_time() into your code -- both implemented here. – Uella

@Uella : The device's count of seconds since Jan 1 2001 occasionally gets corrected backwards by NTP. So it can be non-monotonic on any given device that has a clock crystal that sometimes drifts fast, plus a network connection that can correct this drift. – Authoritarian

I ended up using mach_absolute_time(). I found a little class using it, here. – Donley

@howpaw2 That is a very fine point, you should work that explanation of drifting clocks into your answer. Practically, I could see this showing up as a very strange bug a developer would have a really, really hard time finding, or even repeating. – Uella

Answer 2

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.

Malayoindonesian

Requires #import <QuartzCore/QuartzCore.h>. – Tramontane

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!}

Uella

You forgot to add init = true; in machGetClockS(). – Enki

Not a good test of time resolution, since you tested without wireshark monitoring for any NTP network connections, which can sometimes really mess with the NSDate results. – Authoritarian

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.

Angi

That works perfectly, thanks! I didn't realize NSDate had this level of precision. – Donley