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What is "Frame Sync Time"

Hi people!
Has the frame sync time something to do with the CPU and GPU waiting for each other?

I was testing around with the foliage painting tool. In the attached image you can see two maps: alt text Map in the top: foliage is a mesh with 4 planes | 0.02ms frame sync time
Map in the bottom: foliage is a mesh with 1 plane | 5.65ms frame sync time

both have a triangle count of 2 million and the same lighting conditions
looks like with a single plane as foliage there is just the frame sync time added...

What is happening here?
btw. forward shading and deffered rendering mode show the same effect

Product Version: UE 4.15
foliagetest.jpg (248.2 kB)
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asked May 07 '17 at 03:25 PM in Using UE4

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avatar image izikhuang Feb 05 '18 at 08:20 AM

any update? I also need to know about this Frame Sync Time

avatar image BOBtheROSS Apr 13 '18 at 12:05 AM

I think what is causing the higher rendertime and therefor the game thread and render thread to wait for each other here, is the fact that the GPU has to calculate the transformation (locaiton, rotaion, scale) of the vertex points for every instance. This process needs to happen with reading different values for every single instance, wich is obviously way more often on a single plane than on a bunch of planes to result in the same triangles processed.

This might help with materials on instances too ;) https://answers.unrealengine.com/questions/127435/using-instanced-meshes-doesnt-reduce-draw-calls.html

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The time spent on synchronizing the game thread and renderer thread

> Code Link <

             // this could be perhaps moved down to get greater parallelism
             // Sync game and render thread. Either total sync or allowing one frame lag.
             static FFrameEndSync FrameEndSync;
             static auto CVarAllowOneFrameThreadLag = IConsoleManager::Get().FindTConsoleVariableDataInt(TEXT("r.OneFrameThreadLag"));
             FrameEndSync.Sync( CVarAllowOneFrameThreadLag->GetValueOnGameThread() != 0 );

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answered Feb 05 '18 at 08:32 AM

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Ali Akbar
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avatar image Ali Akbar Feb 05 '18 at 08:34 AM

Looks like with a single grass blade, you have too many draw calls and the game thread waits longer for the render thread to finish drawing all the blades, so it can be synced

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