On 11/30/2012 11:31 PM, Frank Rowand wrote:
On 11/30/12 07:46, Simon Falsig wrote:
[..]
Bonus-question:
- Additionally, I've tried running cyclictest alongside with all the
above, and it actually performs rather well, without any substantial
spikes. A strange thing is though, that the results are actually better
with load than without? (running with -t1 -p 80 -n -i 10000 -l 10000)
- Loaded: Min: 16, Avg: 41, Max: 177
- No load: Min: 16, Avg: 97, Max: 263
If the system is less loaded, then the idle thread might be able to
enter deeper levels of sleep. Deeper levels of sleep have larger
latencies to exit. You would have to look at your processor specific
values for exiting sleep states to see if this is sufficient to explain
the difference.
If running a half-decent version of cyclictest, sleep states are
generally disabled while cyclictest is running. Please watch the line
# /dev/cpu_dma_latency set to 0us
which essentially documents this mechanism. Yes, the name of the
variable "cpu_dma_latency" is not obvious and cyclictest could do a
better job by writing
Wrote 0 to /dev/cpu_dma_latency and keeping the path open to prevent
all cores from entering any sleep state
but this is another story.
A patch that was merged to 3.7 allows to individually enable or disable
sleep states of the ladder governor
(http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commit;h=62d6ae880e3e76098d5e345decd2dce443975889).
It smoothly applies to 3.6-RT as well. This allows to fine-tune the
sleep states by state and core, while the /dev/cpu_dma_latency mechanism
acts on all states and cores, e.g. to disable sleep state 2 and all
deeper states of the ladder governor on core #0, use:
echo 1 >/sys/devices/system/cpu/cpu0/cpuidle/state2/disable
BTW: To analyze how much time a core spent in a specific sleep state,
simply look repeatedly at the "time" variable of a core's sleep state,
e.g. for core #0:
# for i in /sys/devices/system/cpu/cpu0/cpuidle/state[0-4]
> do
> echo -e "`cat $i/name`:\t`cat $i/time`"
> done
POLL: 1342984105
C1-IVB: 737109
C3-IVB: 3852451
C6-IVB: 1702683112
C7-IVB: 4366946606
While cyclictest is running with /dev/cpu_dma_latency set to 0, only the
POLL state times are increasing.
-Carsten.
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