Re: need a timer which can be precise at 1 ms level

Hi,

On 19 Aug 2012, at 17:30, Feixiong Zhang <[email protected]> wrote:

> Hi, all, I'm implementing a network-delay module as a click kernel module.
> What the module does is that when it gets a packets, it will delay the
> packet for a certain time (at millisecond order based on the packet's
> incoming address) and then release it back to host stack. So I need a timer
> that can fire precisely at millisecond level. I test the click timer's
> function schedule_after_msec(1), but the timer actually fires after
> normally 5~7 ms. Is there any other way to get a more precise timer? I took
> a look at the timer document, it says there is a function adjustment()
> which seems can be used. Can someone explain how it works in more detail?
> 
> Besides, what I found for the function timer.schedule_after_msec(1) is if
> there are more frequent packets entering into the delay module, the timer
> firing task will be delayed much more severely. Normally it will fire after
> the incoming packets become sparse, but that is already long time later
> than the time it should fire. Is this normal for click?
> 
> I'm using patchless click on linux 2.6.35-30-generic. I will appreciate a
> lot if anyone can give some hints. Thanks.
> 

Take a look at <http://read.cs.ucla.edu/click/doxygen/class_timer.html#_details>
especially:
"Particularly at user level, there can be a significant delay between a Timer's nominal expiration time and the actual time it runs. Elements that desire extremely precise timings should combine a Timer with aTask. The Timer is set to go off a bit before the true expiration time (see Timer::adjustment()), after which the Task polls the CPU until the actual expiration time arrives."
This is how I would achieve a larger precision too.

best regards,
Bart

-- 
Bart Braem
PATS research group - IBBT
Dept. of Mathematics and Computer Science
University of Antwerp
Campus Middelheim, G3.26
Middelheimlaan 1
B-2020 Antwerpen, Belgium
Phone: +32 (0)3 265.38.64
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Web: www.pats.ua.ac.be