This extension enables an implementation to control the subgroup size by allowing a varying subgroup size and also specifying a required subgroup size.

It extends the subgroup support in Vulkan 1.1 to allow an implementation to expose a varying subgroup size. Previously Vulkan exposed a single subgroup size per physical device, with the expectation that implementations will behave as if all subgroups have the same size. Some implementations may dispatch shaders with a varying subgroup size for different subgroups. As a result they could implicitly split a large subgroup into smaller subgroups or represent a small subgroup as a larger subgroup, some of whose invocations were inactive on launch.

To aid developers in understanding the performance characteristics of their programs, this extension exposes a minimum and maximum subgroup size that a physical device supports and a pipeline create flag to enable that pipeline to vary its subgroup size. If enabled, any SubgroupSize decorated variables in the SPIR-V shader modules provided to pipeline creation may vary between the minimum and maximum subgroup sizes.

An implementation is also optionally allowed to support specifying a required subgroup size for a given pipeline stage. Implementations advertise which stages support a required subgroup size, and any pipeline of a supported stage can be passed a VkPipelineShaderStageRequiredSubgroupSizeCreateInfoEXT structure to set the subgroup size for that shader stage of the pipeline. For compute shaders, this requires the developer to query the maxComputeWorkgroupSubgroups and ensure that:

s = { WorkGroupSize.x × WorkGroupSize.y × WorkgroupSize.z ≤ SubgroupSize × maxComputeWorkgroupSubgroups }

Developers can also specify a new pipeline shader stage create flag that requires the implementation to have fully populated subgroups within local workgroups. This requires the workgroup size in the X dimension to be a multiple of the subgroup size.

Promotion to Vulkan 1.3

Functionality in this extension is included in core Vulkan 1.3, with the EXT suffix omitted. The original type, enum and command names are still available as aliases of the core functionality.

VK_EXT_subgroup_size_control

Name String

VK_EXT_subgroup_size_control

Extension Type

Device extension

Registered Extension Number

226

Revision

2

Extension and Version Dependencies

Version 1.1

Deprecation state

• Promoted to Vulkan 1.3

Contact

• Neil Henning sheredom

Other Extension Metadata

Last Modified Date

2019-03-05

Interactions and External Dependencies

• Promoted to Vulkan 1.3 Core

Contributors

• Jeff Bolz, NVIDIA
• Faith Ekstrand, Intel
• Sławek Grajewski, Intel
• Jesse Hall, Google
• Neil Henning, AMD
• Daniel Koch, NVIDIA
• Jeff Leger, Qualcomm
• Graeme Leese, Broadcom
• Allan MacKinnon, Google
• Mariusz Merecki, Intel
• Graham Wihlidal, Electronic Arts