[LAU] Problems with real time kernel, thermal throttling and cpu freq governors.

[Len Ovens]

On Wed, 24 Sep 2014, Rafael Vega wrote:

> In short, when running the RT kernel from Debian Jessie repos (3.14-2-rt-amd64), the
> scaling governor cannot be changed and the system hangs if the cpu temperature reaches
> the thermal throttling threshold.

It sounds like there are two things that might work... use an older kernel 
(Is there something in 3.14 you must have?, maybe 3.16 will fix if you 
wait.) Set the default governor to performance at compile time (or maybe 
boot time).

Personally, under stress test, I have found that with all cores at 100% 
Performance runs cooler than Ondemand or userspace with some cores set 
lower speed. The only place I find a difference is at idle performance 
seems a bit warmer... but at idle the temperature is way lower anyway.

Is boost turned off in bios? The new kernel needs to have speed step on to 
use Intel's boost technology. Boost over speeds the cpu till it over heats 
then backs off... over simplified yes. Performance may put the chip into 
boost mode (performance is the highest speed after all) all the time. Can 
you get it into userspace? If so you could set the speed manually.

I just checked. Boost speed for your chip is quite high(3.4Gh)

I only have an i5 with 4 cores no HT. (maybe I shouldn't say only as yours 
has only two cores 4 with HT) With the cores set performance and 
stress set to 100% on all cores, I am still 10deg less(~60C) than Max 
running temp. (70C) Temperature shutdown should be a lot higher than that 
(over 100C ... both your chip and mine)). Are you using a temperature 
monitor? The temperature should settle after about 10min at a stress 
level. (both my tests and Intel specs agree on the 10 min thing)

I am using 3.13 lowlatency, but 100% stress should do the same either way. 
It sounds like you machine is running really hot... or is running over 
2.7G I have been using psensor to monitor temperature. Mine is not a laptop though.

Just a note, any testing I have done has shown that running my 3Gh + 
(without boost) CPU at 800Mh (with the userspace governor) gives better 
lowlatency than ondemand at full speed does.

Hyperthreading and ondemand: Ondemand affects even higher latency. HT only 
seems to be an issue for latency below jack at 64/2, at least in my tests.

--
Len Ovens
www.ovenwerks.net