RE: Real-time kernel thread performance and optimization

[Simon Falsig <simon@newtec.dk>]

> -----Original Message-----
> From: Carsten Emde [mailto:C.Emde@osadl.org]
> Sent: 3. december 2012 15:16
> To: frank.rowand@am.sony.com
> Cc: Simon Falsig; linux-rt-users@vger.kernel.org
> Subject: Re: Real-time kernel thread performance and optimization
>
> 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=6
2d
> 6ae880e3e76098d5e345decd2dce443975889).
> 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.

Thanks for the reply! As I wrote in my reply to Frank, I'm not completely
sure if P states are correctly implemented in our system. We're using a
custom BIOS for our custom board, and while P states do show up and are
modifiable (I've currently installed the userspace-governor, and am
manually setting the clock-frequency to the lowest possible at startup),
our board guy is not sure that changing it actually has any effect on the
processor. Yay...:/
So until I get a go-ahead on this, I'll wait with a further investigation.

Best regards,
Simon