On my Thinkpad T440p with a Haswell i7-4700MQ, Fedora 21 was never going below platform c-state PC3, even though the CPU cores and GPU were showing much deeper states (C7 and RC6, respectively). In a default installation, even after tuning things with powertop, it seemed to have terrible idle power draw of 10-12 watts at the dimmest backlight level.
This is using the Intel integrated GPU in Xorg and having the discrete NVIDIA GPU powered down by the nouveau module. (This machine has the displays connected to the Intel GPU and can only use the NVIDIA GPU as an off-screen rendering source.)
Remembering an older tip to save power, I restarted Xorg with 16 bits-per-pixel color depth, and now it is 80% or more in platform c-state PC6 at idle, with an idle power draw around 6-8 watts. Thus, I think either dynamic color depth changes (or ideally: LCD self-sustaining mode to allow the embedded DisplayPort to shut down at idle) will be necessary to get good power savings.
Power management, mobile and firmware developer on Linux. Security developer at Aurora. Ex-biologist. mjg59 on Twitter. Content here should not be interpreted as the opinion of my employer. Also on Mastodon.
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16-bit color depth helps reach lower platform c-states
Date: 2015-05-13 03:11 pm (UTC)This is using the Intel integrated GPU in Xorg and having the discrete NVIDIA GPU powered down by the nouveau module. (This machine has the displays connected to the Intel GPU and can only use the NVIDIA GPU as an off-screen rendering source.)
Remembering an older tip to save power, I restarted Xorg with 16 bits-per-pixel color depth, and now it is 80% or more in platform c-state PC6 at idle, with an idle power draw around 6-8 watts. Thus, I think either dynamic color depth changes (or ideally: LCD self-sustaining mode to allow the embedded DisplayPort to shut down at idle) will be necessary to get good power savings.