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Why would Rust rendering be worse than any other rendering? Rust claims to be well suited for handling parallelism.

by shmerl 1y ago

MindSpunk 1y ago
It is fantastic for CPU parallelism. The problem is the CPU/GPU boundary is difficult to deal with and exposing an API that is both fast and safe and flexible is almost impossible. I don't believe it's possible to make an efficient API at a similar level of abstraction to Vulkan or D3D12 that is safe (as in, not marked unsafe in rust). To do so requires recreating all the complexity of D3D11 and OpenGL style APIs to handle resource access synchronization. The value proposition of D3D12 and Vulkan is that the guardrails are gone and it's up to the user to do the synchronization work themselves. The advantage is that you can make the synchronization decisions at a higher level of abstraction where more assumptions can be made and enforced by a higher-level API. Generally this is more efficient because you can use much simpler algorithms to decide when to emit your barriers, rather than having the driver reverse engineer that high-level knowledge from the low-level command stream. Rust is just not capable of representing the complex interwoven ownership and synchronization rules for using these APIs without mountains of runtime checks that suck away all the benefit of using these APIs. Lots of Vulkan map quite well to Rust's ownership rules, the memory allocation API surface maps very well. But anything that's happening on the GPU timeline is pretty much impossible to do safely. Rust's type system is not sufficiently capable of modeling this stuff without tons of runtime checks, or making the API so awful to use nobody will bother. I've seen GP around a lot and afaik they're using WGPU which is, among other things, Firefox's WebGPU implementation. The abstraction that WebGPU provides is entirely the wrong level to most efficiently use Vulkan and D3D12 style APIs. WebGPU must be safe because it's meant to get exposed to JS in a browser, so it spends a boat load of CPU time to do all the runtime checks and work out the synchronization requirements. Rust can be more challenging here because if you want a safe API you have to be very careful in where you set the boundary between the unsafe internals and the safe API. And Rust's safety rails will be of limited use for the real difficult parts. I'm writing my own abstraction over Vulkan/D3D12/Metal and I've intentionally decided not to make my API safe and to leave it to a higher layer to construct a safe API.
- shmerl 1y ago
  I indeed wouldn't expect safe approach here to be necessarily efficient. But you aren't forced to make everything safe even if it's nicer. I've seen some Vulkan Rust wrappers before which tried to do that, but as you say it comes at some cost. So I'd guess you can always use raw Vulkan bindings and deal with related unsafety and leave some areas that aren't tied to synchronization for safer logic. Dealing with hardware in general is unsafe, and GPUs are so complex that it's sort of expected.
- Animats 1y ago
  > The abstraction that WebGPU provides is entirely the wrong level to most efficiently use Vulkan and D3D12 style APIs. I agree with this, although the WGPU people disagree. There could be a Vulkan API for "modern Vulkan" - bindless only, dynamic rendering only, asset loading on a transfer queue only, multithreaded transfers. That would simplify things and potentially improve performance. But it would break code that's already running and would not work on some mobile devices. We'll probably get that in the 2027-2030 period, as WebGPU devices catch up. WGPU suffers from having to support the feature set supported by all its back ends - DX12, Vulkan, Metal, WebGPU, and even OpenGL. It's amazing that it works, but a price was paid.
- exDM69 1y ago
  > Lots of Vulkan map quite well to Rust's ownership rules, the memory allocation API surface maps very well. But anything that's happening on the GPU timeline is pretty much impossible to do safely. I agree with this, having been dabbling with Vulkan and Rust for a few years now. Destructors and ownership can make a pretty ergonomic interface to the cpu side of gpu programming. It's "safe" as long as you don't screw up your gpu synchronization which is not perfect but it's an improvement over "raw" graphic api calls (with little to no overhead). As for the GPU timeline, I've been experimenting with timeline semaphores. E.g. all the images (and image views) in descriptor set D must be live as long as semaphore S has value less than X. This coupled with some kind of deletion queue could accurately track lifetimes of resources on the GPU timeline. On the other hand, basic applications and "small world" game engines have a simpler way out. Most resources have a pre-defined lifetime, either it lives as long as the application, or the "loaded level" or the current frame. You might even use Rust lifetimes to track this (but I don't). This model is not applicable when streaming textures and geometry in and out of the GPU. What I would really like to experiment with is using async Rust for GPU programming. Instead of using `epoll/kqueue/WaitForMultipleObjects` in the async runtime for switching between "green threads" the runtime could do `vkWaitForSemaphores(VK_SEMAPHORE_WAIT_ANY_BIT)` (sadly this function does not return which semaphore(s) were signaled). Each green thread would need its own semaphore, command pools, etc. Unfortunately this would be a 6-12 month research project and I don't have that much free time at hand. It would also be quite an alien model for most graphics programmers so I don't think it would catch on. But it would be a fun research experiment to try.

Why would Rust rendering be worse than any other rendering? Rust claims to be well suited for handling parallelism.

by shmerl 1y ago

Why would Rust rendering be worse than any other rendering? Rust claims to be well suited for handling parallelism.

It is fantastic for CPU parallelism. The problem is the CPU/GPU boundary is difficult to deal with and exposing an API that is both fast and safe and flexible is almost impossible. I don't believe it's possible to make an efficient API at a similar level of abstraction to Vulkan or D3D12 that is safe (as in, not marked unsafe in rust). To do so requires recreating all the complexity of D3D11 and OpenGL style APIs to handle resource access synchronization. The value proposition of D3D12 and Vulkan is that the guardrails are gone and it's up to the user to do the synchronization work themselves. The advantage is that you can make the synchronization decisions at a higher level of abstraction where more assumptions can be made and enforced by a higher-level API. Generally this is more efficient because you can use much simpler algorithms to decide when to emit your barriers, rather than having the driver reverse engineer that high-level knowledge from the low-level command stream. Rust is just not capable of representing the complex interwoven ownership and synchronization rules for using these APIs without mountains of runtime checks that suck away all the benefit of using these APIs. Lots of Vulkan map quite well to Rust's ownership rules, the memory allocation API surface maps very well. But anything that's happening on the GPU timeline is pretty much impossible to do safely. Rust's type system is not sufficiently capable of modeling this stuff without tons of runtime checks, or making the API so awful to use nobody will bother. I've seen GP around a lot and afaik they're using WGPU which is, among other things, Firefox's WebGPU implementation. The abstraction that WebGPU provides is entirely the wrong level to most efficiently use Vulkan and D3D12 style APIs. WebGPU must be safe because it's meant to get exposed to JS in a browser, so it spends a boat load of CPU time to do all the runtime checks and work out the synchronization requirements. Rust can be more challenging here because if you want a safe API you have to be very careful in where you set the boundary between the unsafe internals and the safe API. And Rust's safety rails will be of limited use for the real difficult parts. I'm writing my own abstraction over Vulkan/D3D12/Metal and I've intentionally decided not to make my API safe and to leave it to a higher layer to construct a safe API.

shmerl 1y ago
I indeed wouldn't expect safe approach here to be necessarily efficient. But you aren't forced to make everything safe even if it's nicer. I've seen some Vulkan Rust wrappers before which tried to do that, but as you say it comes at some cost. So I'd guess you can always use raw Vulkan bindings and deal with related unsafety and leave some areas that aren't tied to synchronization for safer logic. Dealing with hardware in general is unsafe, and GPUs are so complex that it's sort of expected.
Animats 1y ago
> The abstraction that WebGPU provides is entirely the wrong level to most efficiently use Vulkan and D3D12 style APIs. I agree with this, although the WGPU people disagree. There could be a Vulkan API for "modern Vulkan" - bindless only, dynamic rendering only, asset loading on a transfer queue only, multithreaded transfers. That would simplify things and potentially improve performance. But it would break code that's already running and would not work on some mobile devices. We'll probably get that in the 2027-2030 period, as WebGPU devices catch up. WGPU suffers from having to support the feature set supported by all its back ends - DX12, Vulkan, Metal, WebGPU, and even OpenGL. It's amazing that it works, but a price was paid.
exDM69 1y ago
> Lots of Vulkan map quite well to Rust's ownership rules, the memory allocation API surface maps very well. But anything that's happening on the GPU timeline is pretty much impossible to do safely. I agree with this, having been dabbling with Vulkan and Rust for a few years now. Destructors and ownership can make a pretty ergonomic interface to the cpu side of gpu programming. It's "safe" as long as you don't screw up your gpu synchronization which is not perfect but it's an improvement over "raw" graphic api calls (with little to no overhead). As for the GPU timeline, I've been experimenting with timeline semaphores. E.g. all the images (and image views) in descriptor set D must be live as long as semaphore S has value less than X. This coupled with some kind of deletion queue could accurately track lifetimes of resources on the GPU timeline. On the other hand, basic applications and "small world" game engines have a simpler way out. Most resources have a pre-defined lifetime, either it lives as long as the application, or the "loaded level" or the current frame. You might even use Rust lifetimes to track this (but I don't). This model is not applicable when streaming textures and geometry in and out of the GPU. What I would really like to experiment with is using async Rust for GPU programming. Instead of using `epoll/kqueue/WaitForMultipleObjects` in the async runtime for switching between "green threads" the runtime could do `vkWaitForSemaphores(VK_SEMAPHORE_WAIT_ANY_BIT)` (sadly this function does not return which semaphore(s) were signaled). Each green thread would need its own semaphore, command pools, etc. Unfortunately this would be a 6-12 month research project and I don't have that much free time at hand. It would also be quite an alien model for most graphics programmers so I don't think it would catch on. But it would be a fun research experiment to try.