Re: [Xenomai-help] AT91SAM9260 latency

[Juan Antonio Garcia Redondo <juan-antonio.garcia@domain.hid>]

On 28/01/08 14:19, Gilles Chanteperdrix wrote:
> > >
> > >
> > > > > No mystery: hitting a key on a telnet session causes an interrupt
> > > > > masking section of 110us, you see it as the maximum if you never
> > > > > observed longer masking sections, but it is not the maximum if you
> > > > > observed longer masking sections.
> > > >
> > > > OK, but why the masking section on linux side affects to xenomai side ?
> > > > Another thing I don't understand is why when the system has load (above
> > > > I'm talking about calibrator but the same occurs with dd if=/dev/zero
> > > > of=/dev/null), the effect seems to dissapear.
> > >
> > > It is probably not a masking section on linux side but rather a
> > > masking section on I-pipe side. Anyway, the effect does not disappear:
> > > it means that the cache effects cause larger latencies than the
> > > ethernet interrupt, but maybe I did not understand what you explained.
> > > The results you obtain with no load are simply irrelevant.
> >
> > I'll try to explain it better:
> >
> > o Without load I run ./latency -t0 -p500.
> > RTD|      33.182|      53.479|      67.976|       0|      31.250| 77.319
> > RTD|      43.170|      53.479|      67.654|       0|      31.250| 77.319
> > RTD|      41.881|      53.479|      67.332|       0|      31.250| 77.319
> > RTT|  00:02:07  (periodic user-mode task, 500 us period, priority 99)
> >
> > o Each time I press a key (over a telnet session) I can see the lat_max field increase on 40 to 50 us aprox.
> > RTD|      33.505|      53.479|      71.842|       0|      26.739| 77.319
> > RTD|      40.592|      62.177|     123.067|       0|      26.739| 123.067
> >                                    -------
> >                                       \_________: Key pressed
> > RTD|      50.579|      53.479|      73.775|       0|      26.739| 123.067
> 
> This is where you are wrong:
> - first, let me repeat it: test made without load are irrelevant;
I can't agree with you. When we stress a system with load is, as far as
I know, because usually, the large latencies don't appear on a quiet
system. Here we have a case where a large latency (the lat_worst number
I've gotten after more than 7 hours with the system fully loaded is even
less than this) appears on a quiet system and directly related to an
external event.

> - second, an event has no relative effect on max latency, its effect
> is absolute: pressing a key over a telnet session causes, for unknown
> reason,  a masking section of around 130us, which happens to also be
> the worst case latency that we measured properly with a loaded system.
I pointed out the lat_max field because, if you keep the latency test
running and hit a key on a telnet session, you can easily see how the  
lat_max increase each time you hit the key, while the lat_worst increase
depends on the former history.

> 
> Now, if you want to know why you get such a masking section, you are
> free to investigate.

I'll try to do it.

Regards,
	Juan Antonio