The following 'capacity-dmips-mhz' dt property values are used:
Cortex-A15: 1024, Cortex-A7: 539
They have been derived from the cpu_efficiency values:
Cortex-A15: 3891, Cortex-A7: 2048
by scaling them so that the Cortex-A15s (big cores) use 1024.
The cpu_efficiency values were originally derived from the "Big.LITTLE
Processing with ARM Cortex™-A15 & Cortex-A7" white paper
(http://www.cl.cam.ac.uk/~rdm34/big.LITTLE.pdf). Table 1 lists 1.9x
(3891/2048) as the Cortex-A15 vs Cortex-A7 performance ratio for the
Dhrystone benchmark.
The following platforms are affected once cpu-invariant accounting
support is re-connected to the task scheduler:
arndale-octa, peach-pi, peach-pit, smdk5420
The patch has been tested on Samsung Chromebook 2 13" (peach-pi, Exynos
5800).
$ cat /sys/devices/system/cpu/cpu*/cpu_capacity
1024
1024
1024
1024
389
389
389
389
The Cortex-A15 vs Cortex-A7 performance ratio is 1024/389 = 2.63.
The values derived with the 'cpu_efficiency/clock-frequency dt property'
solution are:
$ cat /sys/devices/system/cpu/cpu*/cpu_capacity
1535
1535
1535
1535
448
448
448
448
The Cortex-A15 vs Cortex-A7 performance ratio is 1535/448 = 3.43.
The discrepancy between 2.63 and 3.43 is due to the false assumption
when using the 'cpu_efficiency/clock-frequency dt property' solution
that the max cpu frequency of the little cpus is 1 GHZ and not 1.3 GHz.
The Cortex-A7 cluster runs with a max cpu frequency of 1.3 GHZ whereas
the 'clock-frequency' property value is set to 1 GHz.
3.43/1.3 = 2.64
$ cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_max_freq
1800000
1800000
1800000
1800000
1300000 <-- max cpu frequency of the Cortex-A7s (little cores)
1300000
1300000
1300000
Running another benchmark (single-threaded sysbench affine to the
individual cpus) with performance cpufreq governor on the Samsung
Chromebook 2 13" showed the following numbers:
$ for i in `seq 0 7`; do taskset -c $i sysbench --test=cpu
--num-threads=1 --max-time=10 run | grep "total number of events:";
done
total number of events: 1083
total number of events: 1085
total number of events: 1085
total number of events: 1085
total number of events: 454
total number of events: 454
total number of events: 454
total number of events: 454
The Cortex-A15 vs Cortex-A7 performance ratio is 2.39, i.e. very close
to the one derived from the Dhrystone based one of the "Big.LITTLE
Processing with ARM Cortex™-A15 & Cortex-A7" white paper (2.63).
We don't aim for exact values for the cpu capacity values. Besides the
CPI (Cycles Per Instruction), the instruction mix and whether the system
runs cpu-bound or memory-bound has an impact on the cpu capacity values
derived from these benchmark results.
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
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