[Xenomai] Loose determinism when reading pcap and sending as raw ethernet packets

Gilles Chanteperdrix gilles.chanteperdrix at xenomai.org
Fri Dec 11 15:59:29 CET 2015

On Thu, Dec 10, 2015 at 05:25:41PM +0000, Umair Ali wrote:
> Hello,
> > >
> > >  what you observe is the jitter, you can measure the timer
> > interrupt jitter with the "latency" test, but I bet that maybe the
> > latency may be higher than 5us;
> > - if you really really need to reduce the jitter, then using kernel
> > timers instead of sleeping in a user-space thread is recommended,
> >
> > Yes, you are right that what i have observed is the jitter. I have
> > gone through the links which you have provided me in the last
> > email. I am building my application using POSIX skin. In the
> > links, they have used the rtdm_timer_start(). Now my question is
> > that can i use the rtdm functions in the application compiling
> > with posix skin, if yes then how. Do i have to change the skin for
> > comilation in order to use the rtdm functions. Please refer me
> > some guide for using the Xenomai Kernel timers with posix skin in
> > order to avoid the jitter. Last thing do i need to made a kernel
> > module in order to use the kernel timers. I am confused. Please
> > guide me.
> > If you want to use kernel-space, you need to write a driver,
> > running in kernel spaces, and drivers use the RTDM API. The POSIX
> > API is for the application.
> As i need to send raw Ethernet packets therefore i am using rtnet
> driver for this purpose. Do you mean that i have to write my own
> driver for ethernet card. Or should i have to write an rtdm driver
> that uses rtnet drivers for sending the raw ethernet. Or is there
> a way i can change the rtnet drivers according to my need.

I mean that if you want to send a packet every 5us with very low
jitter, you should not send the packets from a user-space thread,
but pass them to a driver which sends them based on an RTDM timer. I
would not recommend doing that, because you would have to modify
rtnet to do it: the rtnet stack send path may use a mutex, so can
not be called from a timer handler.


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