Re: 2.6.33.7-rt29 PREEMPT_RT worse latency than PREEMPT_DESKTOP on AT91?

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

  On 27/08/10 11:35, Thomas Gleixner wrote:
> On Mon, 23 Aug 2010, Agustin Ferrin Pozuelo wrote:
>> 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
> 33 loops are not really giving you any useful information. Also run
> both tests with some background load and not on a fully idle system.
Sorry, my test setup is not very comprehensive at the moment. I have 
some overnight/overweekend results at hand with millions of loops:

     PREEMPT-DESKTOP:
     root@at91sam9263cpc:~# time cyclictest -i 70000 -p 80 -h 700 -r
     Clock resolution: 0.000000001 (s.ns)
     policy: fifo: loadavg: 0.07 0.09 0.02 1/38 8770
     T: 0 (  980) P:80 I:70000 C:1328241 Min:    135 Act:  218 Avg:  204 
Max:     720

     PREEMPT-RT:
     root@at91sam9263cpc:~# time cyclictest -i 70000 -p 80 -h 700 -r
     Clock resolution: 0.000000001 (s.ns)
     policy: fifo: loadavg: 0.97 1.12 1.18 4/51 31290
     T: 0 (  675) P:80 I:70000 C:3688312 Min:  244    Act:  358 Avg:  
395 Max:     732

Min and avg latencies much worse in PREEMPT-RT on otherwise same setup. 
Worst case similar though.

System is not loaded but many interrupts happen because it is running it 
interactively through ssh+ethernet. (Had to force ethernet to 10Mbit to 
avoid frequent buffer underruns due to sam9263 bug).

I am assuming context switching is more expensive on PREEMPT-RT under 
ARM9, where it seems already a bit expensive.

--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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