Re: RT_PREEMPT and Serial Port

[Arnaldo Carvalho de Melo <acme@redhat.com>]

Em Sun, Apr 13, 2008 at 11:00:08AM -0700, mgross escreveu:
> On Sat, Apr 12, 2008 at 07:34:28PM -0400, Mauricio A. Araya L�pez wrote:
> > On Sat, Apr 12, 2008 at 01:08:37PM -0700, mgross wrote:
> > > On Fri, Apr 04, 2008 at 03:57:53AM -0400, Mauricio A. Araya L�pez wrote:
> > > > Hi all,
> > > > 
> > > > I've been searching on the web (and in the rt-wiki) for the current
> > > > status of the real-time support for devices. For instance
> > > > the serial port, CAN Bus, etc.
> > > > 
> > [...]
> > 
> > > FWIW I was able to get +/- 30usec jitter from a serial port experiment
> > > where I programmed a PIC to push 5 bytes over the serial line every
> > > 20ms. 
> > My results are similar. A 30 usec jitter it seems to be the limit,
> > but indeed the jitter is less, because the latency is near the
> > 10 usec. But, that is "on average", because I've run several
> > test during long time periods, and my worst jitter is like 5 msec!,
> > which is too much. Maybe it is a hardware problem, but I am using
> > no swap, no usb and no DMA, and I dont know which other thing
> > i should disable to not have this huge random jitter.
> > (Obviosly I am locking the memory to have no paging)
> 
> How long did you need to run it to hit a 5msec jitter?
> 
> My tests where only for a few min while I test compiled the kernel a
> few times to see if that would effect things.

And have you guys tried with a ftrace/latency trace enabled kernel? It
would be really interesting to know what happens in this 5ms :-)

- Arnaldo
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