Intel Hardware P-State (HWP) / Intel Speed Shift




Intel Hardware P-State (aka Harware Controlled Performance or "Speed Shift") (HWP) is a feature found in more modern x86 Intel CPUs (Skylake onwards). It attempts to select the best CPU frequency and voltage to match the optimal power efficiency for the desired CPU performance. HWP is more responsive than the older operating system controlled methods and should therefore be more effective.
To test this theory, I exercised my Lenovo T480 i5-8350U 8 thread CPU laptop with the stress-ng cpu stressor using the "double" precision math stress method, exercising 1 to 8 of the CPU threads over a 60 second test run. The average CPU temperature and average CPU frequency were measured using powerstat and the CPU compute throughput was measured using the stress-ng bogo-ops count.
The HWP mode was set using the x86_energy_perf_policy tool (as found in the Linux source in tools/power/x86/x86_energy_perf_policy). This allows one to select one of 5 policies: "normal", "performance", "balance-performance", "balance-power" and "power" as well as enabling or disabling turbo frequencies. For the tests, turbo mode was also enabled to allow the CPU to run at higher CPU turbo frequencies.
The "performance" policy is the least efficient option as the CPU is clocked at a high frequency even when the system is idle and is not idea for a laptop. The "power" policy will optimize for low power; on my system it set the CPU to a maximum of 400MHz which is not ideal for typical uses.
The more useful "balance-performance" option optimizes for good throughput at the cost of power consumption where as the "balance-power" option optimizes for good power consumption in preference to performance, so I tested these two options.
[...]
Running with HWP balance-power option is a good default choice for maximizing compute while minimizing power consumption for a modern Intel based laptop. If one wants to crank up the performance at the expense of battery life, then the balance-performance option is most useful.
The balance-performance option when a laptop is plugged into the mains (e.g. via a base-station) may seem like a good idea to get peak compute performance. Note that this may not be useful in the long term as the CPU frequency may drop back to reduce thermal overrun. However, for bursty infrequent demanding CPU uses this may be a good choice. I personally refrain from using this as it makes my CPU rather run hot and it's less efficient so it's not ideal for reducing my carbon footprint.
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