cpufreq: ondemand: Change the calculation of target frequency

Linux Kernel / Linux Kernel - Stratos Karafotis [semaphore.gr] - 25 July 2013 18:06 EDT

The ondemand governor calculates load in terms of frequency and increases it only if load_freq is greater than up_threshold multiplied by the current or average frequency. This appears to produce oscillations of frequency between min and max because, for example, a relatively small load can easily saturate minimum frequency and lead the CPU to the max. Then, it will decrease back to the min due to small load_freq.

Change the calculation method of load and target frequency on the basis of the following two observations:

- Load computation should not depend on the current or average measured frequency. For example, absolute load of 80% at 100MHz is not necessarily equivalent to 8% at 1000MHz in the next sampling interval.

- It should be possible to increase the target frequency to any
value present in the frequency table proportional to the absolute load, rather than to the max only, so that:

Target frequency = C * load

where we take C = policy->cpuinfo.max_freq / 100.

Tested on Intel i7-3770 CPU @ 3.40GHz and on Quad core 1500MHz Krait. Phoronix benchmark of Linux Kernel Compilation 3.1 test shows an increase ~1.5% in performance. cpufreq_stats (time_in_state) shows that middle frequencies are used more, with this patch. Highest and lowest frequencies were used less by ~9%.

[rjw: We have run multiple other tests on kernels with this change applied and in the vast majority of cases it turns out that the resulting performance improvement also leads to reduced consumption of energy. The change is additionally justified by the overall simplification of the code in question.]

dfa5bb6 cpufreq: ondemand: Change the calculation of target frequency
drivers/cpufreq/cpufreq_governor.c | 10 +--------
drivers/cpufreq/cpufreq_governor.h | 1 -
drivers/cpufreq/cpufreq_ondemand.c | 39 +++++++-----------------------------
3 files changed, 8 insertions(+), 42 deletions(-)

Upstream: git.kernel.org


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