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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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README
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.