max_push_constant_size error

[lonnietc]

Hello,

While I was testing some older hardware, I was able to run the examples from the Rust-GPU Repo (), I did find that the rust-gpu-shadertoy did not work and gave this message:

     Running `target\release\shadertoys-wgpu.exe`
Initialization error: Limit 'max_push_constant_size' value 256 is better than allowed 128

[lonnietc]

Hello,

Just wondering if there is any update on this as the GPU on that testbed is a Radeon RX Vega card (older) and the standard Rust-GPU examples work.

Any update would be greatly appreciated.
Thanks in advance

[LegNeato]

This is set here:

rust-gpu-shadertoys/src/main.rs

Line 109 in 8df4903

max_push_constant_size: 256,

We could probably bump it down if we aren't near the limit.

[lonnietc]

What does this setting affect and will it have any performance impact on better systems?

Thanks again and I will give it a try as soon as I am on that system again which will be when I am back at the office.

[LegNeato]

I don't think so, it is merely that the host needs to declare some value and we chose an arbitrary one. I am fairly sure none of the push constants are that size but haven't double checked.

[lonnietc]

I did some research and read a number online sources on this variable and the Vulkan drivers set a minimal limit of 128 across all hardware. This variable describes the minimum size of the data that is sent to the shaders in the backend.

A summary is here:

The maximum size for push constants is a device-specific limit, but the Vulkan specification guarantees a minimum of at least 128 bytes. Many modern GPUs, particularly NVIDIA and Intel, commonly support 256 bytes, and some niche platforms like Vulkan on macOS may even support up to 4096 bytes. 

The maxPushConstantsSize defines the total amount of high-speed data that can be sent to the shader pipeline in a single command.

The minimum guaranteed size required by the Vulkan specification is indeed 128 bytes. Some older AMD cards adhere strictly to this minimum, while NVIDIA and Intel often support 256 bytes. 

For maximum compatibility of older hardware:

For maximum compatibility with basic, older hardware, especially those that might fall back to OpenGL ES or D3D11 via wgpu's downlevel defaults, relying on push constants is not the safest bet.

If you want to support the widest array of basic and older hardware, you should:

Avoid push constants entirely and use standard uniform buffers instead. This is the most compatible approach because all hardware/backends guarantee robust support for uniform buffers up to a reasonable size (e.g., 64 KiB by default in WebGPU).

As well as some information at:

https://vkguide.dev/docs/chapter-3/push_constants/

I actually adjusted the value to 128 in the main.rs and it seems to run fine on reasonably good hardware, but I will check it on that older system as soon as possible and report back as well.

Thanks again and have a great day

[Firestar99]

Yea Vulkan guaratees 128 bytes, the question is moreso what about other wgpu platforms such as Metal or DX12.

I'm not even sure what push constants are being used for, we could probably replace them with simple buffers quite easily.