Platform/GFX/OffMainThreadCompositing
Current status
Running on Android and Mac, currently fixing Android bugs.
Goals
The main goal is to improve responsiveness. In the longer term, goals include:
- Reduce main thread contention
- Asynchronous scrolling
- Preventing tearing
- Asynchronous video
- Asynchronous CSS animation
Design
A new thread will handle composition. This thread will receive updates from the content thread, much as the chrome process receives updates from the content process in e10s. Indeed, as much as possible, the shadow layers machinery developed for e10s will be adapted for off-main-thread compositing. The design may need to diverge in some places from the e10s approach: for example, on Android, updates from the content thread to the compositing thread need to be asynchronous. (note: With the current code ... this might not be the case with the new frontend, and even in the current code, the cases that require async PLayers could be fixed at the widget/android level too. We'll need to make a tradeoff.)
Plan
Our highest priority platform for this project has been Android, but we want to extend this to all supported platforms: Mac, Windows (D3D9 + D3D10), Linux, B2G (Gonk) and software-only (BasicLayers; possibly, there is ongoing discussion as to whether this is the best use of resources to improve the experience for 'low-end' users).
Current status
Features
- Only scrolling is asynchronous/off main thread. All other features require further work.
Operating systems
- Android: Released. We continue to iron out OMTC bugs as the Android test base expands, but we are already quite high-quality.
- OS X: bug 756601 Alpha quality. We began development of OMTC on this platform, but we haven't worked on it much since then. It pretty much works but is disabled by default.
- Linux: bug 722012 In initial implementation stages. We're still sorting out basic issues with X and GL, as we don't support OMTC with BasicLayers yet.
- B2G (Gonk): bug 741837 Alpha quality. The B2G team has been working on getting it enabled and working through the implementation.
- BasicLayers (software only): bug 703484 In initial implementation stages. Marco Castelluccio, a volunteer contributor, is working through implementing off-main-thread BasicLayers, which also involves making our Cairo library thread-safe.
- Windows:
- Direct3D 10: bug 756606 Not started yet; has some fundamental problems with textures which require the layers refactoring.
- Direct3D 9: bug 756608 Not started yet; but, in theory, should be fairly straightforward (there are already shadow layers implemented, but I don't think they are used anywhere).
Future Work
WebGL
Currently, we read WebGL content back from the graphics card, and then send it to the compositor. This is terrible for performance.
- Sharing WebGL backing textures between threads bug 728524 will let us avoid reading back.
- Double-buffering WebGL bug 716859 will let us hand off a texture to the compositor, then go back to drawing to the WebGL context.
- Not using glFinish() to synchronize between threads bug 697831 (most likely using the ARB_sync extension) will let us hand those textures off faster.
Assignee: Cody Brocious (Daeken)
Status: Being worked
Video
Video decoding is already off-main-thread, but it needs a synchronization point between the Gecko thread and the video thread to publish decoded frames. We need to be able to publish frames directly to the compositor thread, never involving the main thread.
- Asynchronous video composition is bug 706172
Assignee: Nicolas Silva (nical)
Status: Blocked on Linux implementation of OMTC
CSS Animations
Currently CSS animations are driven by the main thread. It'd be better if the style and layout code could assign attributes to relevant layers that would then be interpolated using specified functions on the compositor thread.
- Asynchronous CSS animation is bug 706179
Assignee: David Zbarsky (dzbarsky) Status: Being worked
Plugins
Plugins are already drawn asynchronously (at least on OS X), similarly to how HTML5 video works. Publishing those plugin "frames" to the compositor is directly analagous.
Assignee: UNASSIGNED
Status: No work done
Animated images
Animated images are drawn using the main thread, but at least conceivably we could publish all the frames to the compositor and have the compositor know how to animate the images.
This could help us to animate throbbers, etc, but we don't currently have any layers support at all for animated images, so it will be more work for potentially less gain.
Assignee: UNASSIGNED
Status: No work done
Off-main-thread layers implementations
Features listed above won't work off the main thread unless we have a layers implementation that knows how to draw off the main thread.
Layers refactoring
We are currently refactoring the layers system to more easily facilitate OMTC. The biggest changes are to the compositing side, and to how textures/buffers/handles are passed between the drawing and compositing components. The aim is for most OMTC code to be backend-independent. Compositing will be done by a Compositor class (rather than the ShadowLayerManager, which will still have responsibility for managing the shadow layer tree). There will be backend-specific compositor sub-classes, but the shadow layer tree and it's manager should be backend-independent. Buffers/textures will be handled by a TextureClient/TextureHost pair which will have backend-specific subclasses and will handle all inter-thread communication. The layer classes can then be agnostic about backend and IPC mechanism.
Android
Assignee: Benoit Girard/Ali Juma
Status: Beta quality
Mac OS X
Assignee: UNASSIGNED
Status: Alpha quality
Linux
Assignee: Nicolas Silva
Status: In development
Direct3D 9
Assignee: UNASSIGNED
Status: No work done
Direct3D 10
Assignee: UNASSIGNED
Status: No work done
Basic (Software-only)
This layers implementation is used when we have no hardware-accelerated layers backend; for example, when we're on a system with too-old drivers.
Assignee: Marco Castelluccio
Status: In development
Tracking bug
The tracking bug is bug 598873. Tree view: [1]
Development
Development is happening on mozilla-central and the graphics branch (layers refactoring).