Multiple GPU Queues
[Expose API]: We should expose multiple queues with different capabilities.+
<API Support>: Common across all three APIs (Metal starting with version 2, where `MTLEvent` got introduced). +
<Viable>: Investigation showed that we can safely track the ownership of resources by the queues, and insert the necessary pipeline barriers, making this safe. [Fully Internal]: WebGPU should internally use multiple queues but not necessarily expose them to the user.-
<Ineffective>: WebGPU doesn't know resource dependencies ahead of time, so finding the efficient scheduling route is hard. It's likely to synchronize more than necessary. +
<Metal>: this is what Metal-1 does in the drivers/run-time. [Implicit Transitions]: Ownership transition can happen automatically, based on resources that are used by submissions.+
<Easy>: Code will always work, just not always be concurrent on GPU, even if the developer submits work to different queues. [WebGPU doesn't need fences]: Fences are technically not needed in the API and can be removed?
[Optional Transitions]: Allow the developer to specify an ownership transition ahead of time. [Required Transitions]: Require the developer to transition ownership explicitly.+
<Explicit>: Allows the developer to reason about concurrent GPU processing. Since the developers are not required to use the multiple queues, they want to be sure it works as expected if they do use it. -
<Racy>: Changing the order of submissions may result in a synchronization error. If submissions are triggered by external events (e.g. touch), this may be a hidden even race and a footgun in an otherwise correct application.-
<Logic>: If the order of submission is unexpected, that would likely end up with incorrect rendering anyway. So maybe it's fine to disallow that? <Skip Submit>: on Vulkan, we can avoid submitting an empty command buffer with the semaphore and a pipeline barrier. [Concurrent Textures]: We should support textures that are used concurrently on multiple queues.-
<Cost>: There are clear pefromance costs for having this ability on a texture. It's not able to use specific texture states and layouts, and can't use color compression.