Re: [Xenomai-help] Is xenomai OK for early boot purpose ?

[Philippe Gerum <rpm@xenomai.org>]

On Mon, 2011-01-31 at 12:01 +0100, gryma biloy wrote:
> On Mon, Jan 31, 2011 at 11:53 AM, Philippe Gerum <rpm@xenomai.org> wrote:
> > On Mon, 2011-01-31 at 11:51 +0100, Gilles Chanteperdrix wrote:
> >> Philippe Gerum wrote:
> >> > On Mon, 2011-01-31 at 09:55 +0100, gryma biloy wrote:
> >> >> Hello,
> >> >>
> >> >> On a project running Linux on a beagle board, I'd like to add the
> >> >> following capabilities to my platform :
> >> >>   - real time processes with periodic tasks (10ms, 100ms & 200 ms) and
> >> >> a low jitter (< 50 us)
> >> >
> >> > Kernel space apps then, likely. Userland apps (the recommended way)
> >> > probably have a larger worst-case jitter on this hardware, albeit still
> >> > below 100 us. Gilles?
> >>
> >> Las time I checked, latency measured on a 720MHz omap3 was around 55us
> >> with a 1ms period and 35us with a 100us period.
> >
> > Yeah, I suspect that longer periods ( >= 10 ms ) may introduce more
> > opportunities for cache eviction due to linux activity, and therefore
> > higher jittery, even if bounded.
> 
> Does this mean that the main reason for jittery is due to the cache ?
> What would be the other reasons, if any ?


The main reason for more jittery with longer periods is due to the cache
if you compare a high frequency loop and a lower one. When the rt system
is idle, waiting for the next wakeup, the linux activity resumes and
evicts a bunch of cachelines for running its own processes. So next time
the rt system is resumed, a significant time will be spent refilling the
cache with the rt program, starting with some real-time IRQ handler
itself. This is why you always get better latencies when running a 10Khz
sampling loop, than you would get for 1Khz. This can be observed on all
archs Xenomai supports.

On ARM, the memory subsystem is often involved in jittery for different
reasons (ask Gilles why he implemented FCSE for instance).

> 
> 
> 
> >> >>   - early boot operations (i.e. being able to perform some tasks asap
> >> >> after a power on reset)
> >> >>
> 
> >> >
> >> > I don't see how Xenomai could directly help in any way for the early
> >> > boot goal. If you question is about whether Xenomai initializes fast
> >> > enough, and early enough during the linux boot process for kicking rt
> >> > applications as soon as possible, then the answer is yes, that should
> >> > do.
> >>
> >> Well, user-space applications will not get started before user-space is
> >> started, i.e. basically before root filesystem has been mounted and init
> >> is running. You could write your real-time application as the init
> >> application, but that would be kind of awkward.
> 
> 
> Understand. I will make a try a see if I can meet some of my constraints.
> 
> Thanks for your replies,
> 

-- 
Philippe.