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
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