Re: [Xenomai] Performance impact after switching from 2.6.2.1 to 2.6.4

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On Tue, Jun 28, 2016 at 01:45:59PM +0200, Wolfgang Netbal wrote:
> 
> 
> Am 2016-06-28 um 12:39 schrieb Gilles Chanteperdrix:
> > On Tue, Jun 28, 2016 at 12:31:42PM +0200, Wolfgang Netbal wrote:
> >>
> >> Am 2016-06-28 um 12:19 schrieb Gilles Chanteperdrix:
> >> min: 10, max: 677, avg: 10.5048 -> 0.0265273 us
> >>
> >> Here are the output for Kernel 3.0.43 and Xenomai 2.6.2.1
> >>
> >> #> ./tsc
> >> min: 10, max: 667, avg: 11.5755 -> 0.029231 us
> > Ok. So, first it confirms that the two configurations are running
> > the processor at the same frequency. But we seem to see a pattern,
> > the maxima in the case of the new kernel seems consistently higher.
> > Which would suggest that there is some difference in the cache. What
> > is the status of the two configurations with regard to the L2 cache
> > write allocate policy? Could you show us the tsc results of Xenomai
> > 2.6.4 with the 3.0 kernel ?
> As requested I created a Kernel 3.0.43 with Xenomai 2.6.4
> #> dmesg | grep "Linux version"
> Linux version 3.0.43 (netwol@DSWUB001) (gcc version 4.7.2 (GCC) ) #186 
> SMP PREEMPT Tue Jun 28 13:28:40 CEST 2016
> 
> #> dmesg | grep "Xenomai"
> [    0.844697] I-pipe: Domain Xenomai registered.
> [    0.849188] Xenomai: hal/arm started.
> [    0.853189] Xenomai: scheduling class idle registered.
> [    0.858350] Xenomai: scheduling class rt registered.
> [    0.882246] Xenomai: real-time nucleus v2.6.4 (Jumpin' Out) loaded.
> [    0.888926] Xenomai: starting native API services.
> [    0.893811] Xenomai: starting RTDM services.
> 
> #> dmesg | grep I-pipe
> [    0.000000] I-pipe 1.18-13: pipeline enabled.
> [    0.331174] I-pipe, 396.000 MHz timer
> [    0.334900] I-pipe, 396.000 MHz clocksource
> [    0.844697] I-pipe: Domain Xenomai registered.
> 
> 
> Here the output of tsc
> min: 10, max: 345, avg: 10.5 -> 0.0265152 us

Ok, so 3.0.43 with 2.6.4 has the same consistent behaviour with
regard to the __xn_rdtsc() latency as 3.0.43 with 2.6.2.1. So:

- if you find 2.6.4 with 3.0.43 slower than 3.0.43 with 2.6.2.1, you
can remove the kernel version change from the mix and do your tests
from now on exclusively with the kernel 3.0.43, and the tsc latency
is unlikely to be the cause of the performance difference. To really
make sure of that, you can replace __xn_rdtsc() in tsc.c with a call
to rt_timer_tsc(), recompile and rerun on the two remaining
configurations.

- if you find 2.6.4 with 3.0.43 is as fast as 3.0.43 with 2.6.2.1,
you can remove the Xenomai version change from the mix and do you
from now on exclusively with Xenomai 2.6.4. And the question about
differences in cache configuration remains.

-- 
					    Gilles.
https://click-hack.org