Off Main Thread Painting

OMTP, or ‘off main thread painting’, is the component of Gecko that allows us to perform painting of web content off of the main thread. This gives us more time on the main thread for javascript, layout, display list building, and other tasks which allows us to increase our responsiveness.

Take a look at this blog post for an introduction.


Painting (or rasterization) is the last operation that happens in a layer transaction before we forward it to the compositor. At this point, all display items have been assigned to a layer and invalid regions have been calculated and assigned to each layer.

The painted layer uses a content client to acquire a buffer for painting. The main purpose of the content client is to allow us to retain already painted content when we are scrolling a layer. We have two main strategies for this, rotated buffer and tiling.

This is implemented with two class hierarchies. ContentClient for rotated buffer and TiledContentClient for tiling. Additionally we have two different painted layer implementations, ClientPaintedLayer and ClientTiledPaintedLayer.

The main distinction between rotated buffer and tiling is the amount of graphics surfaces required. Rotated buffer utilizes just a single buffer for a frame but potentially requires painting into it multiple times. Tiling uses multiple buffers but doesn’t require painting into the buffers multiple times.

Once the painted layer has a surface (or surfaces with tiling) to paint into, they are wrapped in a DrawTarget of some form and a callback to FrameLayerBuilder is called. This callback uses the assigned display items and invalid regions to trigger rasterization. Each nsDisplayItem has their Paint method called with the provided DrawTarget that represents the surface, and they paint into it.

High level

The key abstraction that allows us to paint off of the main thread is DrawTargetCapture 1. DrawTargetCapture is a special DrawTarget which records all draw commands for replaying to another draw target in the local process. This is similar to DrawTargetRecording, but only holds a reference to resources instead of copying them into the command stream. This allows the command stream to be much more lightweight than DrawTargetRecording.

OMTP works by instrumenting the content clients to use a capture target for all painting 2 3 4 5. This capture draw target records all the operations that would normally be performed directly on the surface’s draw target. Once we have all of the commands, we send the capture and surface draw target to the PaintThread 6 where the commands are replayed onto the surface. Once the rasterization is done, we forward the layer transaction to the compositor.

Tiling and parallel painting

We can make one additional improvement if we are using tiling as our content client backend.

When we are tiling, the screen is subdivided into a grid of equally sized surfaces and draw commands are performed on the tiles they affect. Each tile is independent of the others, so we’re able to parallelize painting by using a worker thread pool and dispatching a task for each tile individually.

This is commonly referred to as P-OMTP or parallel painting.

Main thread rasterization

Even with OMTP it’s still possible for the main thread to perform rasterization. A common pattern for painting code is to create a temporary draw target, perform drawing with it, take a snapshot, and then draw the snapshot onto the main draw target. This is done for blurs, box shadows, text shadows, and with the basic layer manager fallback.

If the temporary draw target is not a draw target capture, then this will perform rasterization on the main thread. This can be bad as it lowers our parallelism and can cause contention with content backends, like Direct2D, that use locking around shared resources.

To work around this, we changed the main thread painting code to use a draw target capture for these operations and added a source surface capture 7 which only resolves the painting of the draw commands when needed on the paint thread.

There are still possible cases we can perform main thread rasterization, but we try and address them when they come up.

Out of memory issues

The web is very complex, and so we can sometimes have a large amount of draw commands for a content paint. We’ve observed OOM errors for capture command lists that have grown to be 200MiB large.

We initially tried to mitigate this by lowering the overhead of capture command lists. We do this by filtering commands that don’t actually change the draw target state and folding consecutive transform changes, but that was not always enough. So we added the ability for our draw target capture’s to flush their command lists to the surface draw target while we are capturing on the main thread 8.

This is triggered by a configurable memory limit. Because this introduces a new source of main thread rasterization we try to balance setting this too low and suffering poor performance, or setting this too high and suffering crashes.


OMTP is conceptually simple, but in practice it relies on subtle code to ensure thread safety. This was the most arguably the most difficult part of the project.

There are roughly four areas that are critical.

  1. Compositor message ordering

    Immediately after we queue the async paints to be asynchronously completed, we have a problem. We need to forward the layer transaction at some point, but the compositor cannot process the transaction until all async paints have finished. If it did, it could access unfinished painted content.

    We obviously can’t block on the async paints completing as that would beat the whole point of OMTP. We also can’t hold off on sending the layer transaction to IPDL, as we’d trigger race conditions for messages sent after the layer transaction is built but before it is forwarded. Reftest and other code assumes that messages sent after a layer transaction to the compositor are processed after that layer transaction is processed.

    The solution is to forward the layer transaction to the compositor over IPDL, but flag the message channel to start postponing messages 9. Then once all async paints have completed, we unflag the message channel and all postponed messages are sent 10. This allows us to keep our message ordering guarantees and not have to worry about scheduling a runnable in the future.

  2. Texture clients

    The backing store for content surfaces is managed by texture client. While async paints are executing, it’s possible for shutdown or any number of things to happen that could cause layer manager, all layers, all content clients, and therefore all texture clients to be destroyed. Therefore it’s important that we keep these texture clients alive throughout async painting. Texture clients also manage IPC resources and must be destroyed on the main thread, so we are careful to do that 11.

  3. Double buffering

    We currently double buffer our content painting - our content clients only ever have zero or one texture that is available to be painted into at any moment.

    This implies that we cannot start async painting a layer tree while previous async paints are still active as this would lead to awful races. We also don’t support multiple nested sets of postponed IPC messages to allow sending the first layer transaction to the compositor, but not the second.

    To prevent issues with this, we flush all active async paints before we begin to paint a new layer transaction 12.

    There was some initial debate about implementing triple buffering for content painting, but we have not seen evidence it would help us significantly.

  4. Moz2D thread safety

    Finally, most Moz2D objects were not thread safe. We had to insert special locking into draw target and source surface as they have a special copy on write relationship that must be consistent even if they are on different threads.

    Some platform specific resources like fonts needed locking added in order to be thread safe. We also did some work to make filter nodes work with multiple threads executing them at the same time.

Browser process

Currently only content processes are able to use OMTP.

This restriction was added because of concern about message ordering between APZ and OMTP. It might be able to lifted in the future.