Mir's default DisplayBuffer clone mode forces all outputs to the rate of the slowest one (and increases latency too)

[Daniel van Vugt]

Clone mode forces all outputs to render at the rate of the slowest one.

This is unnecessary. If you're cloning between say a 24Hz TV and a 60Hz laptop then Mir should be smart enough to give them separate compositors so as to keep the laptop screen rendering at 60Hz.

[Daniel van Vugt (vanvugt)]

Oops, forgot it's defaulting to clone mode so the compositing rate is indeed throttled to the slowest output (24Hz).

Setting the outputs side-by-side forces Mir to create separate compositors:
    bin/mir_demo_server_shell --display-config=sidebyside
so now they're independent and the laptop display renders smoothly at 60Hz.

[Daniel van Vugt (vanvugt)]

I think it would be helpful to make the decision of whether outputs share a single DisplayBuffer an option. Because people might want a faster smoother clone mode, or a single display buffer that spans all outputs (for efficiency or to force a stubborn shell to span). Our default heuristic doesn't let the user choose this right now.

[Daniel van Vugt (vanvugt)]

This bug is getting annoying. It means I can't plug an external monitor into a laptop and easily measure its latency using mirvanity. Because the default DisplayBuffer cloning increases latency by an extra frame or so.

[Michał Sawicz (saviq)]

Syncing task from Mir.