Comments on: Units of mach_absolute_time()

By: orion

orion — Sat, 23 Apr 2011 21:50:01 +0000

great post, thank you.
i was noticing that something i was animating based on CACurrentMediaTime was subtly jerky and staccato, so i replaced CACurrentMediaTime with a wrapper around mach_absolute_time and it’s much better. this is iPad + raw openGL. my guess is that altho CACurrentMediaTime is a double, it actually only has millisecond accuracy, which then bumps into aliasing on the 60Hz render timer.

By: Justin

Justin — Sun, 19 Apr 2009 05:59:55 +0000

I just checked on my G4 PowerBook running 10.5 – its exactly 30 nanoseconds per tick. So its never been guaranteed to be 1:1.

Win32 has a similar function “QueryPerformanceFrequency()”. Even different Intel/AMD systems have different frequencies, depending on BIOS settings of the various motherboard manufactures. More information here:
http://msdn.microsoft.com/en-us/library/bb173458.aspx

So unless your code is guaranteed to be only running on your own personal mac, please use mach_timebase_info() – that’s what its for :-)

By: Julia

Julia — Sun, 12 Apr 2009 01:20:51 +0000

mach/mach_time.h

By: Julia

Julia — Sun, 12 Apr 2009 01:19:47 +0000

Great information! Thanks!

And don’t forget to:

#include

By: Devin Lane

Devin Lane — Fri, 27 Mar 2009 00:39:47 +0000

A good suggestion. While the values are documented to be 1:1 on Mac OS X on all systems up until now, and this will likely continue into the future, code that needs more portability may benefit from using mach_timebase_info as you suggest.

By: Justin

Justin — Sat, 28 Feb 2009 11:03:10 +0000

I believe you should always use the values in mach_timebase_info, because they could be different from system to system depending on the host CPU and bus, even just on Mac OS X.

By: Nathan

Nathan — Sun, 19 Oct 2008 02:41:00 +0000

Thanks! No telling how many hours you just saved me.

By: Jesse Towner

Jesse Towner — Sat, 11 Oct 2008 22:51:50 +0000

Thanks for the information.