Energy Efficiency¶
This document aims to explore in depth the energy efficiency considerations around Mir based compositors.
Note that Mir is a C++ library for building compositors, not a product itself. The following applies to all Mir based compositors, but it is theoretically possible to defeat the available efficiencies in such products.
GPU acceleration¶
Mir based compositors default to hardware compositing on any available GPU. Mir includes a “graphics platform” abstraction that allows efficient implementation on different software or graphics stacks.
Display timeout¶
Mir provides an idle-timeout configuration option for setting the time (in
seconds) the compositor will remain idle before turning off the display.
Compositor optimizations¶
Occlusion¶
Where client windows are “occluded” when they do not contribute to the content of the display. For example, being offscreen or hidden by another window.
Mir does not trigger recomposition when the content of an occluded window updates. It also drastically “throttles” the repainting of the occluded windows.
Hardware composition¶
Depending on the capabilities of the GPU composition of visual elements (such as cursors and application windows) can be handled without direct intervention of the CPU or the explicit provision of a full frame buffer.
Historically, Mir development has focused on hardware with limited capabilities for hardware composition. However, both the range of hardware for which Mir is considered and the capabilities of “low end” hardware are increasing. As a result, Mir has work in progress to exploit additional features of hardware composition options where they are available.
Mir currently uses hardware composition for cursors (where supported by the GPU hardware) and can, in some usecases, enable “composition bypass” for fullscreen clients.
Damage tracking¶
Damage tracking involves passing information about which parts of buffers have been updated. That can avoid the need to regenerate parts of the display that have not changed.
Mir has not implemented this optimisation yet.
Metrics¶
We are unable to provide meaningful metrics for power consumption attributable to Mir as actual power consumption is dependent on these variables:
the CPU, GPU and memory characteristics of the system
the number, size and configuration of the displays
the number of application windows being composited
the rate at which buffers are submitted to the application windows
the Mir configuration deployed in the product