[PlanetCCRMA] kernel-rt latency issues

Fernando Lopez-Lezcano nando at ccrma.Stanford.EDU
Wed Nov 7 09:58:32 PST 2012

On 10/31/2012 08:28 AM, Rob Wallace wrote:
> Recently I’ve been evaluating the real-time PREEMPT_RT patch. I noticed
> that PlanetCCRMA already had RPM packages so I thought I would try them out.
> When I ran the ‘cyclictest’ on the ccrma kernel I noticed a lot of
> latency and jitter. This didn’t seem normal compared to the PREEMPT_RT
> kernel I built myself. After investigating the issue I found certain
> kernel options in the ccrma kernel that should not be selected for a
> real-time system.

Hi Rob and thanks for the feedback! (sorry for the delay in getting back 
to you, I was on a trip and then moving to a new house with very little 
time to pack, very very busy).

Do you have any results from the tests you did that compare your build 
and the one that I do for Planet CCRMA? I do use our builds for audio 
work and they seem to be working mostly fine.

As Tracey already mentioned I try to get the kernels I build to be as 
close as possible in terms of the options selected to the stock Fedora 
kernels (mostly for the sake of compatibility - you should be able to 
boot either and not run into any problems or unexpected behavior).

>These are the kernel options I changed to fix the
> latency issue in your build:
> # CONFIG_SUSPEND is not set

Aha... these two would probably have to stay. I imagine most Planet 
CCRMA users would be interested at least in having suspend available, I 
do use that all the time in my laptop.

> # CONFIG_PM_RUNTIME is not set
> # CONFIG_PM_DEBUG is not set

I could probably do away with these two. That would lead to a rise 
(perhaps?) in power drain. It would be interested to know how much of a 
difference this makes, if any.


I don't know why these are set up as "y" in the Fedora kernel...

> # CONFIG_CPU_FREQ is not set
> # CONFIG_INTEL_IDLE is not set

Hmmm, again, probably I would have to keep these. So this affects 
latency even if you set the CPU governor to performance? (or set all 
CPUs to max speed with cpupower?)

> You can try a before and after test to see the difference. I ran the
> cyclictest as follows:
> cyclictest -a2 -m –n -I -i 100 -p 80

I'll look into these options in more detail when I have some free cycles...

It could be also an option to have a second build that is 
super-optimized for latency and darn the compatibility but I don't know 
if there would be much demand for it. Feedback welcome!

-- Fernando

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