Desktop compositing latency is real

Re:Desktop compositing latency is real by pcwalton on Tuesday November 21, 2017 @12:12PM (#15749448)

The issue with sending a tree of layers is that this prevents some important optimizations: avoiding overdraw with early Z becomes impossible, because your app doesn't know anything about the positions of the scrollable layers and so has to be conservative and paint all of their contents. So you trade a frame of latency for lots of overdraw, which is a tradeoff I'm not really comfortable making. (Note that today, almost all apps overdraw like crazy anyway, but that should be fixed.) :)

> Further, Microsoft engineers have said that hardware overlays are coming, in which the graphics card pulls content from the windows as it's sending the video out the port, rather than taking an extra frame time to paint the windows onto a composited surface.

This seems like the best solution to me. It allows apps to paint content intelligently to prevent overdraw while avoiding any latency.

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Re:Desktop compositing latency is real by raphlinus on Tuesday November 21, 2017 @01:24PM (#15750246)

It might be lots of overdraw in the general web case, but for a text editor almost everything the app renders is going to be shown on the screen. The exception is a bit of stuff just outside the scroll viewport, so it can respond instantly to scroll requests. This feels like a good tradeoff to me.

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Re:Desktop compositing latency is real by CyberDildonics on Tuesday November 21, 2017 @12:23PM (#15749566)

> So you trade a frame of latency for lots of overdraw, which is a tradeoff I'm not really comfortable making.

I would jump at the chance to make that trade. My 8 year old CPU is rarely taxed by normal usage. What's the point in having a faster computer if it feels slower?

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Re:Desktop compositing latency is real by roel_v on Wednesday November 22, 2017 @03:51AM (#15755351)

Overdraw doesn't just use some CPU. If that would be all, it'd be an easy problem to solve. It causes extra data to be transferred over various buses, extra stops in a pipeline of operations that is already cramped trying to provide a frame every few ms, and probably other problems that I don't know about. This problem isn't as easy as 'throw some more CPU at it'.

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Re:Desktop compositing latency is real by Const-me on Wednesday November 22, 2017 @08:17AM (#15756509)

I think it’s only that complex and taxing for GUI frameworks that mix CPU and GPU rendering. Like Win32 GDI or GTK+.

For modern GUI frameworks that were designed for GPU, like MS WPF/XAML or Mozilla WebRender, overdraw just use some GPU resources, but that’s it. GPUs are designed for much more complex scenes. In-game stuff like particles, lights, volumetric effects, and foliage involve tons of overdraw. In comparison, a 2D GUI with overdraw is very light workload for the hardware.

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Re:Desktop compositing latency is real by pcwalton on Wednesday November 22, 2017 @11:27AM (#15758052)

Overdraw actually matters quite a bit for integrated GPUs, especially on HiDPI. On high-end NVIDIA or AMD GPUs, sure, 2D overdraw doesn't matter too much. But when power is a concern, you don't want to be running on the discrete GPU, so it's worth optimizing overdraw.

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Re:Desktop compositing latency is real by CyberDildonics on Wednesday November 22, 2017 @08:25AM (#15756557)

That seems like a dubious claim to me. In my experience maxing out memory or pcie bandwidth is rarely a bottleneck, and extremely unlikely / impossible with low CPU usage.

Do you have a link or more detail about 'already cramped pipelines'?

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Re:Desktop compositing latency is real by pcwalton on Wednesday November 22, 2017 @11:27AM (#15758055)

2D rendering on GPUs is almost entirely memory bound.

Source: I've been working on GPU 2D rendering full time for years now.

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Re:Desktop compositing latency is real by jreck on Wednesday November 29, 2017 @03:35AM (#15805485)

Depends heavily on the GPU and the scene being rendered. On Android I've seen low-end GPUs bottlenecked by the shader ALU of all things, even though the 2D renderer only produces trivial shaders. Turns out it's easier to whack off shader compute cores than it is to muck with the memory bus in some cases.

Most common situation is the GPU isn't taxed at all, though, and doesn't even leave idle clocks. We end up just being limited by GL draw call overhead.

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Re:Desktop compositing latency is real by CyberDildonics on Thursday November 23, 2017 @01:46PM (#15767052)

You seem to be implying that rendering of a text editor has to be all 2d and that rendering a frame pushes the memory bandwidth to it's limits, both of which can't possibly be true. Why can games run at 144hz and above but a text editor can't afford to overdraw for decreased latency?

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