Spinning Spheres demo is no longer fluid

[jdm]

./mach run --release https://cdn.rawgit.com/servo/servo.org/gh-pages/bhtml-newtab/webrender-demos/spheres.html no longer feels smooth in Servo.

[glennw]

I did a few tests and it doesn't appear to be rendering related. Of note:

• The render threads and GPU times are well under the 60 fps threshold (16.67ms).
• The frame rate is > 60 fps (with vsync disabled), but still a lot lower than the renderer is running at (perhaps limited by layout / style?).
• The jank itself seems unrelated to the framerate, since it is running above 60 fps.

[boustrophedon]

This demo appears to render the same for Servo and Chromium for me. The spheres are "wobbly" in both, though somewhat more in Servo for some reason. Firefox has an extremely low framerate for some reason.

I'd be interested in debugging this if someone can tell me what I'm looking for. Also, can I just do a git bisect inside the servo directory or do I have to do something more because of mach?

[jdm]

A git bisect should work.

[boustrophedon]

Okay! So what am I looking for? Less wobbly spheres?

[jdm]

I'm not sure what you mean by wobbly, but in a release build last July the motion was very fluid.

[boustrophedon]

I'll take a video of what I see on my computer, because it seems like we might be seeing different things.

[boustrophedon]

Here is a video showing what I see with checkout at 5dac1f6, from the last bors commit I found in July.

Here is a video showing what I see with checkout at 26de7c6, that I checked out yesterday.

Both were recorded with recordMyDesktop at 50fps because for some reason it doesn't want to record above that.