Clock and Timing Granularity

Clock and Timing Granularity presents ideas for a Tcl clock with finer granularity.

How precise is the timing clock in Tcl? When using after, an interesting thing happens when you start using small values, you don't necessarily get the callback until later. This can cause problems if you are doing animation, like I am right now.

Run on different WinXP SP2 computers:

Not so good.

% set e [clock micro] ; after 5 {puts [expr [clock micro] - $e]}

Now this is more like it!

% set e [clock micro] ; after 5 {puts [expr [clock micro] - $e]}

Different machines will fail at different small values. I think this is because of the way after must be implemented on windows. See Results of some quick research on timing in Win32 , Ryan Geiss, 2002-08-16 (...with updates since then).

How to fix it?

I'm thinking to make a separate thread use a better Win32 timing schedule command and call the main thread. I guess this is reimplementing after. Anyone have a better idea?

From the web

As I mentioned earlier the default PIC resolution for the OS when it starts up is around 16 milliseconds. Let’s say that you set a periodic timer to fire every 5 milliseconds, with the PIC set at 16 milliseconds you will only be alerted every 16 milliseconds (at best). This level of accuracy is usually good enough for most applications. However, for time critical applications such as audio and video playback this resolution just is not good enough.
Why are the Multimedia Timer APIs (timeSetEvent) not as accurate as I would expect? , James Dailey, 2009-07-02

See Also

clock click -milliseconds resolution - again... , comp.lang.tcl, 2004-06-02
in which KBK discusses the extent Tcl goes to to get finer granularity, and what is problematic about Windows