Firefox's WebRender has mixed results for me on Linux

November 25, 2020

I wrote last week about how WebRender introduced bad jank in my Linux Firefox under some circumstances. However, it turns out that WebRender for me has mixed results even outside of that issue, as I reported on Twitter:

[...] In the bad news, the WebRender Firefox is clearly less responsive on CSS hovers on than the regular one.

(The specific issue I see is that if I wave the mouse up and down the page, the hover highlight can visibly lag behind the mouse position a bit. With WebRender off, this doesn't happen. The laggy performance shows up clearly in the Performance recordings in Web Developer tools, where I can see clear periods of very low FPS numbers and the overall average FPS is unimpressive.)

This is on my home machine, which has integrated Intel graphics (on a decent CPU) and a HiDPI screen. Today I was in the office and so using my office machine, which uses a Radeon RX 550 graphics card (because it's an AMD machine and good AMD CPUs don't have onboard GPUs) and dual non-HiDPI screens, and in very light testing my Firefox was using WebRender and didn't seem as clearly laggy on CSS hovers on as my home machine.

(This isn't quite a fair test because my office machine isn't running quite as recent a build of Nightly as my home machine is.)

At one level, this is unsurprising. On Linux, WebRender has long had block and allow lists that depended both on what sort of graphics you had and what screen resolution you were running at (this was in fact one of the confusing bits of WebRender on Linux, since Firefox didn't make it clear what about your setup was allowing or stopping WebRender). Presumably Mozilla has good reason for these lists, in that how well WebRender performed likely varies from environment to environment, or more exactly from some combination of GPU and resolution to other combinations.

At another level, this is disappointing. Firefox's WebRender is supposed to be a great performance improvement, delivering smooth 60 FPS animation (presumably including CSS effects), but in practice some combination of Firefox WebRender, the Linux X11 graphics stack, and my specific hardware results in clearly worse results than the old way. All of that effort on everyone's part has delivered an outcome that makes me turn off WebRender and plan to ignore it until I have no other choice. This is especially personally disappointing because WebRender is a necessary enabler for things like hardware accelerated video playback.

(I have to confess that I've held my nose and turned to Chrome for the single job of displaying a couple of sites where I really care about smooth video performance. I use Chrome Incognito windows for this, which at least limits some of the damage. I still hold my views on walking away from Chrome, but I'm a pragmatist.)

Written on 25 November 2020.
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Last modified: Wed Nov 25 00:18:15 2020
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