Hello! I'm curious if anyone here has a good idea about interleaving works between a compute shader and a fragment shader.

Some relevant details:

• My app is built with Rust and wgpu, and I'm running on an M1 Macbook Pro.
• I have a single encoder with a compute pipeline and a render pipeline.
• The compute shader writes to a storage buffer defined like this:

@group(0) @binding(2) var<storage, read_write> output: array<vec4<f32>>;

• The fragment shader reads from the same buffer. Basically, each fragment is just one element of the vec4<f32> . The fragment shader is very simple, and doesn't touch anything else in the storage buffer.

I've added timestamp queries to the pipeline, and what I'm seeing is this:

Duration #1: 47.800208ms

Duration #2: 47.809876ms

Frame time: 51.2545ms

Duration #1 is computed from the compute shader timestamps (the duration between the beginning and end of the compute pass) and Duration #2 is the time for the render pass, computed the same way.

Frame time is measured on the CPU.

I expected the duration of the compute shader and fragment shader to add up to the frame time (approximately). But it doesn't and I'm confused about why! Could it be due to interleaving of the compute pass and render pass? If so, I'm curious how the synchronization works. How does the GPU figure out the dependencies between the write (a compute shader invocation) and the reader (fragment shader invocation)?

I don't have any explicit synchronization, but I'm also not seeing any tearing or anything that would indicate that there is a data race between the shaders.