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Optimizing descriptor sets and layouts for Vulkan

Midgard and Bifrost family Mali GPUs support four simultaneous bound descriptor sets at the API level. However, they require a single physical descriptor table per draw call internally.

Prerequisites

You must understand the following concepts:

  • Descriptor sets.
  • Binding spaces.

Descriptor sets and layouts

If any of the four source descriptor sets have changed, then the driver rebuilds the internal table for a draw call.

The first draw call, after a descriptor changes, has a higher CPU overhead than following draw calls that reuse the same descriptor set. Larger descriptor sets cause a more expensive rebuild.

With current drivers, the descriptor set pool allocations are not pooled. Do not call vkAllocateDescriptorSets() on a performance critical code path.

How to optimize descriptor sets and layouts

Try using the following optimization steps:

  • Pack the descriptor set binding space as much as possible.
  • Instead of resetting descriptor pools and reallocating new descriptor sets, update descriptor sets that are already allocated, but no longer referenced.
  • Reuse pre-allocated descriptor sets and do not update them with the same information every time.
  • Use VK_DESCRIPTOR_TYPE_UNIFORM_BUFFER_DYNAMIC or VK_DESCRIPTOR_TYPE_STORAGE_BUFFER_DYNAMIC to bind the same UBO or SSBO with different offsets. The alternative is building more descriptor sets.

Descriptor set and layout steps to avoid

Arm recommends that you:

  • Do not leave holes in the descriptor set.
  • Do not leave unused entries as copying and merging still has a computational cost.
  • Do not allocate descriptor sets from descriptor pools on performance critical code paths.
  • Do not use DYNAMIC_OFFSET UBOs/SSBOs if you never plan on changing the binding offset, as there is a small, extra, cost for handling the dynamic offset.

The negative impact of inefficient descriptor sets and layout

Unoptimized Vulkan descriptor sets and layout leads to a risk of increased CPU load for draw calls.

Debugging your descriptor sets and layout performance problems

Ways to speed up your debugging process:

  • Monitor the pipeline layout for unused entries.
  • Monitor for contention-related performance problems on vkAllocateDescriptorSets().