WebGPU Internals - Bind Group Cache

This class implements a cache of GPU bind groups to avoid recreating them each frame.

Cache implementation

The cache is a node tree:

class WebGPUBindGroupCacheNode {
    public values: { [id: number]: WebGPUBindGroupCacheNode };
    public bindGroups: GPUBindGroup[];

    constructor() {
        this.values = {};
    }
}

export class WebGPUCacheBindGroups {
    private static _Cache: WebGPUBindGroupCacheNode = new WebGPUBindGroupCacheNode();
    ...
}

The id key in the values object is the ID of a bind group resource: a uniform or storage buffer, a sampler, or a texture. The id value for the uniform or storage buffer and texture is simply the uniqueId property of the corresponding class (DataBuffer.uniqueId and InternalTexture.uniqueId / ExternalTexture.uniqueId, respectively). For the sampler, it is the sampler hash code, computed by WebGPUCacheSampler.GetSamplerHashCode(). The cache is traversed and built by looping over all the buffers, samplers, and textures used by a shader, which is encapsulated in a WebGPUPipelineContext, in that order.

Limits of the implementation

The location of a resource, meaning the group and binding indices in the [[group(G), binding(B)]] syntax, is not factored into the ID, and the IDs are not globally unique because they do not come from the same pool: there is a separate pool for the buffer and texture uniqueId property. So, theoretically, collisions could occur where two sets of resources point to the same cache entry. In practice, this is very unlikely to happen. Making the cache foolproof would also make it slower, and the WebGPU implementation already pays a significant cost for managing caches for some objects, mainly bind groups and render pipelines.

Also note that all uniform buffers have an offset of 0 in Babylon, and we do not have a use case where the same buffer would be used with different capacity values. That means we only need to take the buffer ID into account in the cache, not its offset or size.

Optimization

The cache includes an optimization that simply returns the existing bind groups if the draw and material contexts have not changed since the last cache query. The draw context holds the list of uniform and storage buffers, and the material context holds the list of textures and samplers used by the shader. If those lists have not changed, the previously created bind groups are still valid.

Monitoring the performances

Cache performance can be assessed by looking at these properties. Each property should be prefixed by BABYLON.WebGPUCacheBindGroups.: