I can change sampling_rate to 40000 with the 2.6.28-12 kernel, but not
the 2.6.30-rc7 kernel. Values equal to or above 80000 are fine.
This behaviour looks to be deliberate in 2.6.30 - cpufreq_ondemand.c has
this in it:
/* Above MIN_SAMPLING_RATE will vanish with its sysfs file soon
* Define the minimal settable sampling rate to the greater of:
* - "HW transition latency" * 100 (same as default sampling / 10)
* - MIN_STAT_SAMPLING_RATE
* To avoid that userspace shoots itself.
*/
static unsigned int minimum_sampling_rate(void)
{
return max(def_sampling_rate / 10, MIN_STAT_SAMPLING_RATE);
}
so the kernel must be calculating a minimum of 80000 for my processor
(Intel(R) Core(TM)2 Duo CPU T9300 @ 2.50GHz). It's the same on another
PC with a Pentium M @ 1.7 GHz.
The main reason I've been looking at changing from the defaults is that
after rebooting, I get good performance, but after running for some
hours, the system becomes reluctant to switch quickly to max freq (my
test case is running a game under wine; it runs typically at 90-120% CPU
as reported by top, and even with up_threshhold set to 80, the CPU
mostly stays stuck at 800MHz, and the frame rates suck). I get this with
the stock kernel as well as the 2.6.30 kernel.
--
CPU Scaling too aggressive
https://bugs.launchpad.net/bugs/107545
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