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
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