2.6.33.7-rt29 PREEMPT_RT worse latency than PREEMPT_DESKTOP on AT91?

[Agustin Ferrin Pozuelo <agustin.ferrin@cgglobal.com>]

Hello,

I am fine tuning the configuration for an ARM system derived from 
AT91SAM9263-EK.

My goal is to minimize latency, and I am using "cyclictest" from 
rt-tools v0.78 for measuring it.

I get consistently better latency with PREEMPT_DESKTOP over what I get 
with PREEMPT_RT. This is an example for a very simple test run, which 
reflects the overall results I am getting:

    ### PREEMPT-RT, HRT, no NO_HZ, no RTC, no USE_SLOW_CLOCK

    root@at91sam9263cpc:~# time cyclictest -i 700000 -r -p 80 -l 33
    Clock resolution: 0.000000001 (s.ns)
    policy: fifo: loadavg: 0.11 0.10 0.04 1/49 884

    T: 0 (  861) P:80 I:700000 C:     33 Min:    370 Act:  627 Avg:  437 Max:     627
    real	0m 23.51s
    user	0m 0.38s
    sys	0m 2.20s

    ### PREEMPT-DESKTOP (no RT), HRT, no NO_HZ, no USE_SLOW_CLOCK
    root@at91sam9263cpc:~# time cyclictest -i 700000 -r -p 80 -l 33
    Clock resolution: 0.000000001 (s.ns)
    policy: fifo: loadavg: 0.24 0.08 0.03 1/46 600

    T: 0 (  574) P:80 I:700000 C:     33 Min:    173 Act:  196 Avg:  222 Max:     378
    real	0m 23.66s
    user	0m 0.32s
    sys	0m 1.29s
       

That is, I get much better min, avg, and max latency with 
"PREEMPT_DESKTOP". How can this be?

Anyway, the results seem worse than the <150us worst case) obtained with 
2.6.24.7-rt27 by Remy Bohmer 
<https://rt.wiki.kernel.org/index.php/CONFIG_PREEMPT_RT_Patch#Platforms_Tested_and_in_Use_with_CONFIG_PREEMPT_RT>, 
though he was apparently/ not using 'Tickless and HRT' /because of CPU 
usage issues... But without those I can't get cyclictest to run properly!

My guess is that the RT patch adds code (checks, locks) to may critical 
sections that happens to be specially slow on ARM9...

Clues & comments welcome!
--Agustín.

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Agustín Ferrín Pozuelo

Embedded Systems Engineer

CG Power Systems Ireland Limited

Automation Systems Division

Herbert House, Harmony Row, Dublin 2, Ireland.

Phone: +353 1 4153700 Web: www.cgglobal.com <http://www.cgglobal.com>

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