It seems like using DWMFlush causes us to wait for the DWM to finish it's work before starting any of ours.

Chrome has started using WaitForVBlank and there's a thread here with piles of info:
https://bugs.chromium.org/p/chromium/issues/detail?id=953970

https://cs.chromium.org/chromium/src/ui/gl/vsync_thread_win.cc?q=WaitForVBlank&sq=package:chromium&dr=C&l=115

See also the complaints here: https://www.vsynctester.com/firefoxisbroken.html

Switching to WaitForVBlank can buy us 1ms+ of frame budget which can be really important on lower end machines.

We originally ran into problems with WaitForVBlank acquiring a process wide lock but it seems like those problems were limited to Win7.

From https://bugs.chromium.org/p/chromium/issues/detail?id=953970:
"1. Use WaitForVBlank in Win8+ only, there is a process wide lock in Win7 there prevents other DX from running in the process if you call WaitForVBlank."

This seems to work without issues on my dual monitor setup, and substantially reduces the instability in the frame-to-frame delay as measured via QueryPerformanceCounter: from a full millisecond above or below the 16.66 we want, to less than 0.2ms of wobble. Both the peaks and the general variation seems improved.

I haven't tested yet what happens during sleep, monitor time-out, laptop close, etc. I've attached the patch will all the timing scaffolding as part of it. I could take it all out and make a proper review if we think this is enough?

There's also a bit in there where I simply assign vsync = now when using VBlank, and that seems to drop the blue/purple lines on vsynctester.com by about 2ms; not entirely sure what that means but it sounds like a good thing? :)

It would probably be good to make this controllable via a pref. That way we can get it in early and have people easily change it if they run into problems.

Intel HD630 @_@

https://hg.mozilla.org/mozilla-central/rev/d05d35839c2a

I had a look at a new gpuview trace with this change it looks like it has the benefit that we're looking for. We start doing work ~1ms earlier.

Comment on attachment 9139642 [details]
Bug 1628137 - Switch to using WaitForVBlank for vsync on Windows

Beta/Release Uplift Approval Request

• User impact if declined: This relatively simple change has shown a substantial performance improvement in a number of situations that we've tested. We'd like to get it out as soon as possible.

This change doesn't actually enable the new vsync method (it's behind a pref) but will enable wider testing.

• Is this code covered by automated tests?: No
• Has the fix been verified in Nightly?: No
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): There should be no behaviour change because the code is behind a pref.
• String changes made/needed:

Enable by default on Windows 10, unless force disabled by pref.

Comment on attachment 9139642 [details]
Bug 1628137 - Switch to using WaitForVBlank for vsync on Windows

Lands this new functionality between an off-by-default pref for wider opt-in testing. Approved for 76.0b5.

https://hg.mozilla.org/releases/mozilla-beta/rev/8138cfc9f25f