Off-main-thread compositing on Linux

We recently enabled off-main-thread compositing on Linux. As I write this post it is enabled on Firefox nightly and Firefox developer edition and if nothing bad comes up it will ride the trains and get enabled in beta and in the stable release of Firefox 40.

Web browsers render web content in separate intermediate surfaces (we call them layers) in order to be able to efficiently animate, scroll or zoom some elements independently of the others. Compositing is the action of taking these layers and flattening them in one surface that can be brought to the screen.

Gecko used to render and composite layers from the same thread (the main thread), which is also responsible for a lot of expensive operations such as running JavaScript and computing the layout of the page. This meant that if something was taking too long on the main thread, it would also block/delay everything else.

But some operations can be performed independently of one another. For instance, a video decoder should be able to send video frames to the screen without being blocked by a script taking too long. To some extent, scrolling and zooming should also be performed without being blocked by a script that contains an infinite loop. Here comes off-main-thread compositing (let’s save a few key strokes and call it OMTC from now on).

With OMTC, Gecko does the compositing work on a dedicated thread. This thread is never waiting for the main thread, so we can make sure it runs smoothly. This lets us, for example, have video decoding threads talk directly to the compositor which improves video playback dramatically. This also opens the door to a lot of optimizations that we have been building on other platforms, but could not on Linux because they were relying on OMTC’s architecture. Chief among them, asynchronous panning and zooming (APZ), letting us perform scrolling and zooming on the compositor thread at 60 frames per seconds even when the main thread can’t keep this frame rate up (teaser: APZ is a real game changer). There is also asynchronous animations which (no surprise there) lets animations be performed by the compositor, again with 60 frames per second no matter how busy the main thread is. Currently, async animations are enabled on Linux but APZ is still in the work.

OMTC has been around for a long while. In fact we have shipped it on mobile platforms for years, but it took a long while before we managed to enable it by default on Mac, then Windows and finally on Linux recently. APZ is already enabled on mobile platforms and is getting close to ship on Mac and Windows, Linux should come soon after.

This is very exciting for the gfx team, because we have been working on getting OMTC enabled for a long time now. This is the base for a lot of important optimizations and will let us finally simplify a lot of code since we now use the same compositing architecture on all of our platforms. While this change is not obvious to user, it is a – hidden but huge – step towards great things to come, so there will be a lot to be excited about moving forward (although don’t expect everything to happen tomorrow or the day after, these things take time!).

OMTC on Linux, in its current state is still pretty new and comes with some improvements and regressions we are confident that we will be able to catch up with all of the regressions eventually but, as usual, don’t hesitate to file bugs at bugzilla.mozilla.org.

Another thing to be excited about is the work to move from Gtk 2 to Gtk 3. This is a long-standing project and I will blog about it later, when we get closer to enabling it by default. I am very pleased to see progress in this area and I’ll use this post as an opportunity to thank Martin Stránský, Lee Salzman, and the many others who are helping making this happen.