Shaders and descriptor sets
This is part 3 of a blog series I’m writing on Granite‘s Vulkan backend. In this episode we are looking at how we deal with shaders and descriptor sets. At this point in our design process, there are many, many choices to make. Especially descriptor sets need to be carefully considered.
Hash all the things
A theme we start to see now is hashmaps and lazy creation of objects. One thing you run into with Vulkan’s pipeline-related types are how much work it is to be explicit all the time. The amount of information we need to provide is staggering. I believe it not healthy for mind and soul to work at low levels here except in special cases, and we should aggressively hide away detail where we can. There is naturally a clock cycles vs. sanity tradeoff to be made here.
You can argue that the lines between high-level GL/D3D11-style design and Granite’s model are quite blurred. The (mental) price to pay to be explicit is just not worth it in my opinion. I will try to explore the obvious alternatives here and provide more context why the design is the way it is.
04 – Shaders and pipeline layouts
The first step in creating a pipeline is of course, to create a VkShaderModule from our SPIR-V code. This is a no-brainer, but next we need a pipeline layout, which in turn requires VkDescriptorSetLayouts. The sample is here https://github.com/Themaister/Granite-MicroSamples/blob/master/04_shaders_and_programs.cpp.
Rather than manually declaring the pipeline layout like a caveman I think using reflection to automatically generate layouts is a good idea. There is no reason for users to copy information which exists in the shaders already. For the reflection, I use SPIRV-Cross. If we never need to compile SPIR-V in runtime (game engine scenario), there is no reason why we cannot shift the reflection step to off-line as well and just pass the side-band data along to remove a runtime dependency. I never got as far as building a nice off-line SPIR-V baking pipeline, so I just compile GLSL on the fly with shaderc. However, the interface in the Vulkan backend just consumes raw SPIR-V.
A common mistake beginners tend to do is to think that names are important in binding interfaces. This is a mistake carried over from the GL and D3D11 days. The only things we should care about are descriptor sets, bindings and location decorations as well as push constant use. This is the only semantic information we need to create binding interfaces, i.e. pipeline layouts and pipelines.
A pipeline layout in Vulkan needs to know all shader stages a-la GL programs, so we also need a step to combine shaders into a Vulkan::Program. Here we take the union of reflection information and request handles for Vulkan::DescriptorSetAllocator and Vulkan::PipelineLayout. This is hashed, but there is no performance concern here since we should do all of this work in load time when possible anyways. These handles are all owned internally in Vulkan::Device, and there is no reason to worry about object lifetime for these objects.
I don’t think there is a reason to deviate far from this design unless you have a very specific scheme in mind with descriptor set allocation. As I’ll explore later, using bindless descriptors extensions or explicit descriptor set allocation could motivate use of a “standard” pipeline layout, in which case reflection gets kind of meaningless anyways.
05 – The binding model – embracing laziness
I never really had a problem with the old-school way of binding resources to binding slots. It just isn’t the part of the old APIs I felt were lacking, so Granite is kind of old school here, but it does have full consideration for descriptor sets and I removed any impedance mismatch with Vulkan (i.e. no translation needed to bridge between Granite and Vulkan). E.g.:
cmd->set_storage_buffer(set, binding, *resource); cmd->set_texture(set, binding, resource->get_view(), Vulkan::StockSampler::LinearClamp);
The old binding models in GL/D3D11 have flat binding spaces with no separation of per-frame, per-material, or per-draw bindings. In Granite I wanted to take full advantage of the descriptor set feature where we can assign some kind of “frequency” and relation between bindings. Here is an example to illustrate how it is used: https://github.com/Themaister/Granite-MicroSamples/blob/master/05_descriptor_sets_and_binding_model.cpp.
In draw time, we can use the current pipeline layout and pull in the binding points which are active and make sure we bind descriptor sets with the correct resources. This is actually hot code, so I spent time designing a nice system here which tries to be as optimal as possible, given these restrictions.
Because of mobile, we need some conservative limits. I use 4 descriptor sets and 16 (dense) binding points per set (minimum spec of Vulkan). This allows for fairly compact pipeline layout descriptions, and we can loop over bitsets to look at resources. This is also just fine for my use cases.
When it comes to allocation of descriptor sets themselves, I think I have a very different approach to most. A Vulkan::DescriptorSetAllocator is represented as:
- The VkDescriptorSetLayout
- A bunch of VkDescriptorPools which can only allocate VkDescriptorSets of this set layout. Pools are added on-demand.
- A pool of unused VkDescriptorSets which are already allocated and can be freely updated.
- A temporary hashmap which keeps track of which descriptor sets have been requested recently. This allows us to reuse descriptor sets directly. In the ideal case, we almost never actually need to call vkUpdateDescriptorSets. We end up with hash -> get VkDescriptorSet -> vkCmdBindDescriptorSets. When a descriptor set has not been used for a couple of frames (8), we assume that it is no longer relevant, and the set is recycled, and some other descriptor set can reuse it and just call vkUpdateDescriptorSet. We definitely do not want to keep track of when any buffer or image resources is destroyed, and recycle early. That’s tracking hell which slows everything down.
The temporary hashmap is a data structure I’m quite happy with. It’s used for a few other resources as well. See https://github.com/Themaister/Granite/blob/master/util/temporary_hashmap.hpp for the implementation.
On certain GPUs, allocating descriptor sets is, or at least used to be very costly. The descriptor pools might not be implemented as true pools (sigh …), so every vkAllocateDescriptorSets would mean a global heap allocation, absolutely horrible for performance. This is the reason I’m not a big fan of the “one large pool” design. In this model, we just allocate a massive VkDescriptorPool, and we just allocate from that, for any kind of descriptor set. This means recycling VkDescriptorSet handles over many frames is impractical. The intended use pattern is to call vkResetDescriptorPool and allocate new descriptor sets which are only valid for one frame at a time, just like command buffers. There is also the problem of knowing how to balance the descriptor load for these massive pools, what’s the ratio of image descriptors vs uniform buffer descriptors, etc. With per-descriptor set layout allocators, there is zero guess work involved.
Alternative design – Bindless
Bindless is all the rage right now. The only real complaint I have is that it’s only supported on desktop and requires an EXT extension. It also means writing shaders in a very specific way. I don’t really need it for my use cases, but bindless enables certain complex algorithms which benefit from accessing a huge set of resources dynamically.
Alternative design – persistent explicit VkDescriptorSets
An alternative is exposing descriptor sets directly and only allow users to bind descriptor sets rather than individual resources. The API user would need to build the sets manually. While this is an idea, I think there are too many hurdles to make it practical.
- We need to know and declare the target imageLayout of textures up front. This is obvious 99% of the time (e.g. a group of material textures which are SHADER_READ_ONLY_OPTIMAL), but in certain cases, especially with depth textures, things can get rather ambiguous. This does seem to me like an API design fault. It is unclear why this information is needed.
- Some resources are completely transient in nature and it does not make sense to place them in persistent descriptor sets. The perfect example here is uniform buffers. In later samples, we’ll look at the linear allocator system for transient data.
- Some resources depend on the frame buffer, i.e. input attachments. Baking descriptor sets for these resources is not obvious, since we need to know the combination pipeline layout + frame buffer, which should have nothing to do with each other.
- We need to know the descriptor set layout (and by extension, the shaders as well) up-front. This is problematic if resources are to be used in more than one shader. The common fix here is to settle on a “standard” pipeline layout so we can decouple shaders and resources. This usually means a lot of padding and redundant descriptor allocations instead. We have a limited amount of descriptor sets when targeting mobile (4). We do not have the luxury of splitting every individual “group” of resources into their own sets, some combinatorial effects are inevitable, making persistent descriptor sets less practical. On desktop, 8 sets is the norm, so that might be something to consider.
- Hybrid solutions are possible, but complexity is increased for little obvious gain.
I’m happy with my design. It’s very easy to use, but there is a CPU prize I’m willing to pay and I honestly never saw it in the profiler. I think resource binding models are cases where shaving overhead away will shave your sanity away as well, at least if you want to be compatible with a wide range of hardware. It’s much easier if you only cater to high-end desktop where bindless can be deployed.
… up next!
Next up we will explore the linear allocators for uniform, vertex, index and staging data.