Re: boot time, process start time, and NOW time

[George Anzinger <george@mvista.com>]

James Courtier-Dutton wrote:
> Albert Cahalan wrote:
> 
>>>>
>>>>
>>>> That's userspace, which works fine on a 2.4.xx kernel.
>>>> If userspace were to change, it wouldn't work OK for
>>>> a 2.4.xx kernel anymore. So consider that cast in stone.
>>>>
>>>> "now" is the time() function. Using gettimeofday()
>>>> would only make sense if I decided to pay the cost
>>>> of asking for the time every time I look at a task.
>>>>
> 
> While on the subject of time, is it possible to get a monotonic timer 
> with 1ms or better resolution?
> We need this for linux multimedia applications, and it is used to sync 
> audio and video. Currently we use gettimeofday(). If a movie is playing, 
> and the user goes and changes the time, or changes the timezone, we do 
> not want that to effect the movie playing. I have not been able to find 
> a monotonic 1ms accurate timer in the linux kernel, that is available to 
> applications, and has little overhead. Some efficient ioctl or function 
> call for uptime to 1ms accuracy would do perfectly.

If all you want is the time try
clock_gettime(CLOCK_MONOTONIC, struct time_spec *tv)

Should work fine on 2.6.x kernels.  This is good to what ever the fine structure 
is on the box, e.g. TCP cycles on most x86 or pm_timer cycles on some, in any 
case it is good to better than a micro second.
> 
   If you want a timer, look into the posix clocks & timers which were added at 
2.6.
-- 
George Anzinger   george@mvista.com
High-res-timers:  http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml