The confusion of multithreaded rendering

[guodong chen]

My program will render in 2 threads,

1. Can Dawn's single Queue submit buffers from multiple threads?

2. If different threads create different devices, can resources be shared across different devices?

[Corentin Wallez]

1) If you externally synchronize access access to the rest of the Dawn resources yes. But in general no. Dawn hasn't been made to work well when used multithreaded yet.

2) no.

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[Dominic Cerisano]

This topic came up when discussing distributed render/compute multiqueues a couple of years ago.

Dominic Cerisano

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[guodong chen]

If I wanted to use an Offscreen Canvas style, could Dawn draw the texture in the A thread and then use it in the B thread？

[Corentin Wallez]

Not at the moment. Dawn is very much single threaded for now. The only thing that's safe multithreaded is command recording. We expect to improve this in the future but it hasn't been necessary for the Web part of WebGPU so it hasn't been the priority.

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[guodong chen]

I checked Dawn's interfaces. Which interfaces are used for recording?

[Austin Eng]

wgpu::CommandEncoder, wgpu::RenderPassEncoder, wgpu::ComputePassEncoder, wgpu::RenderBundleEncoder are the recording interfaces.

The only thing safe to do multithreaded is after making a command encoder, record certain types of commands on it on another thread, and then call CommandEncoder::Finish() to get a wgpu::CommandBuffer. You need to synchronize access to the queue if you want to submit the wgpu::CommandBuffer on the main thread.

Types of commands you should not record multithreaded:

 - Indirect draw commands - that will not be thread-safe since internally it allocates extra buffers.

 

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[guodong chen]

If different devices cannot share resources, is it safe for a device to allocate resources including the Buffer Texture pipeline in different threads?

[Corentin Wallez]

Only command encoding is safe, resource creation with buffer/texture/bindgroups etc isn't (unless you manually lock the device), nor is queue submit etc.

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[guodong chen]

It feels like Dawn has been around for a long time, is it stable enough to be used in commercial projects