[yocto] Yocto Realtime tests on beaglebone black

[Stephen Flowers]

So I ran cyclictest with an idle system and loaded with multiple 
instances of cat /dev/zero > /dev/null &

#cyclictest -a 0 -p 99 -m -n -l 100000 -q

I ran this command as shown by Toyoka at the 2014 Linuxcon Japan 
[http://events.linuxfoundation.org/sites/events/files/slides/toyooka_LCJ2014_v10.pdf]
to compare against his results for the BBB.  I also threw in xenomai 
with kernel 3.8 for comparison.  For the standard kernel HR timers were 
disabled.

[idle]
preempt_rt: min 12 avg: 20 max: 59
standard: min: 8005 avg: 309985955 max: 619963985
xenomai: min: 8 avg: 16: max 803

[loaded]
preempt_rt: min 16 avg: 21 max: 47
standard: min: 15059 avg: 67769851 max: 135530885
xenomai: min: 10 avg: 15: max 839

Actually the preempt_rt results tie up pretty well with Toyooka above, 
leading me to conclude theres something off in my code that could be 
optimised - what do you guys think.
Also, I ran a test with preempt_rt at 100Hz and there was maybe 10% 
improvement in latency.

Steve

On 12/02/2015 00:35, William Mills wrote:
> + meta-ti
> Please keep meta-ti in the loop.
>
> [Sorry for the shorting.  Thunderbird keep locking up when I tried 
> replay all in plain text to this message.]
>
> ~ 15-02-11, Stephen Flowers wrote:
> > Thanks for your input.  Here are results of 1000 samples over a
> > 10 second period:
> >
> > Interrupt response (microseconds)
> > standard: min: 81, max:118, average: 84
> > rt: min: 224, max: 289, average: 231
> >
> >Will share the .config later once I get on that machine.
>
> Steve I agree the numbers look strange.
> There may well be something funny for RT going on for BBB.
> TI is just starting to look into RT for BBB.
>
> I would like to see the cyclictest results under heavy system load for
> standard and RT kernels.  The whole point of RT is to limit the max
> latency when the system is doing *anything*.
>
> I am not surprised that the standard kernel has good latency when idle.
> As you add load (filessystem is usually a good load) you should see 
> that max goes up a lot.
>
> Also, as Bruce says, some degradation of min and average and also
> general system throughput is expected for RT.  That is the trade-off.
> I still think the number you are getting for RT seem high but I don't
> know what your test is doing in detail.  (I did read your explanation.)
> cyclictest should give us a standard baseline.
>
>
> On 02/11/2015 10:25 AM, Bruce Ashfield wrote:
>> On 15-02-11 03:50 AM, Stephen Flowers wrote:
>>>
>>> my bad, here is the patch set.
>>> As for load, only system idle load for the results I posted previously.
>>> Will run some cyclic test next.
>>
>> One thing that did jump out was the difference in config_hz, you
>> are taking a lot more ticks in the preempt-rt configuration. If
>> you run both at the same hz, or with no_hz enabled, it would be
>> interesting to see if there's a difference.
>>
>> Bruce