Featherweight Musings

Wednesday, August 29, 2012

Layers refactoring

OMTC is one of the major performance wins for Firefox at the moment and we want to bring it to all platforms. The major sub-system involved in OMTC is Layers, in particular the shadow layers system (which I blogged about a while back). Unfortunately, it is a bit of a mess at the moment, and it is very hard to reuse code. The Layers system design is very elegant and extendable, and I like it a lot, but the extension to multiple process (or threads) is not the greatest. Bas Schouten and Ali Juma came up with a major redesign for the whole system, which was just lovely, but unfortunately we don't have the resources to work on that, and it seems extremely difficult to do such a major refactoring without being disruptive or getting stuck in an endless cycle of rebasing.

So, we are going to do a slightly less ambitious refactoring, mostly of the shadow classes which work on the compositing thread. Bas and Ali came up with the design, Ali started it off, and I am now carrying the baton. The goal is to be able to implement OMTC on a platform with as little extra layers code as possible. We separate out the shadow layer manager into a layer manager and a compositor. The layer manager takes care of the layer hierarchy and the compositor is responsible for the graphical composition of quads. There will be one compositor per backend, but only only one shadow layer manager. Any backend specific code in the shadow layers will be moved into the compositor, so there will only be one shadow layer of each kind (e.g., Thebes layer, container layer).

The method of sharing graphics memory (for example, textures for the accelerated backends) are backend dependent and will be moved out of the layers classes. We will introduce the concept of texture hosts and texture clients. Texture hosts reside on the compositor thread, and clients on the drawing thread. There will be pairs for each method of sharing a buffer: shared memory, OpenGL textures, Direct3D textures, and so forth. Obviously not all host types will have implementations on all backends, but potentially there could be multiple implementations of each kind of texture host. Texture clients are only used by basic layers, and there is only one implementation for each kind of 'texture' (but, for e.g., OpenGL textures and D3D textures count as different kinds of texture client).

Texture host/clients are designed to be very lightweight, they only manage graphical memory and the communication between host and client processes (most of which is actually done outside of those classes, anyway). I think we also need a higher level, heavier weight abstraction. At the moment (in the refactoring) these are called image hosts and image clients, but the name will probably change. These 'images' represent some buffer of graphical data, which could be a single texture or it could be in YUV form - with one texture for each colour channel - or could be an image made up of many texture tiles. I haven't figured out exactly how this will work yet. These images should be backend independent (they will use different texture host/clients on different backends), and it would be nice to share as much code as possible between layer types, so, for example, canvas layers can use the same image host/client as image layers. There is a lot of duplication there at the moment.

The compositor/layer manager work is mostly done, and the texture host/client work is on its way. The image host/client is also on its way, but the design is evolving as I go along. You can follow on the graphics branch, if you are interested. I will blog some more as the design solidifies.

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