Name

VK_EXT_subgroup_size_control - device extension

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

• Requires Vulkan 1.1

Contact

• Neil Henning https://github.com/KhronosGroup/Vulkan-Docs/issues/new?title=VK_EXT_subgroup_size_control:%20&body=@sheredom%20

Other Extension Metadata

Last Modified Date

2019-03-05

Contributors

• Jeff Bolz, NVIDIA
• Jason 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

Description

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 PipelineShaderStageRequiredSubgroupSizeCreateInfoEXT 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:

\[s = { WorkGroupSize.x \times WorkGroupSize.y \times WorkgroupSize.z \leq SubgroupSize \times maxComputeWorkgroupSubgroups }\]

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.

New Structures

• Extending PhysicalDeviceFeatures2, DeviceCreateInfo:

  • PhysicalDeviceSubgroupSizeControlFeaturesEXT

• Extending PhysicalDeviceProperties2:

  • PhysicalDeviceSubgroupSizeControlPropertiesEXT

• Extending PipelineShaderStageCreateInfo:

  • PipelineShaderStageRequiredSubgroupSizeCreateInfoEXT

New Enum Constants

• EXT_SUBGROUP_SIZE_CONTROL_EXTENSION_NAME

• EXT_SUBGROUP_SIZE_CONTROL_SPEC_VERSION

• Extending PipelineShaderStageCreateFlagBits:

  • PIPELINE_SHADER_STAGE_CREATE_ALLOW_VARYING_SUBGROUP_SIZE_BIT_EXT
  • PIPELINE_SHADER_STAGE_CREATE_REQUIRE_FULL_SUBGROUPS_BIT_EXT

• Extending StructureType:

  • STRUCTURE_TYPE_PHYSICAL_DEVICE_SUBGROUP_SIZE_CONTROL_FEATURES_EXT
  • STRUCTURE_TYPE_PHYSICAL_DEVICE_SUBGROUP_SIZE_CONTROL_PROPERTIES_EXT
  • STRUCTURE_TYPE_PIPELINE_SHADER_STAGE_REQUIRED_SUBGROUP_SIZE_CREATE_INFO_EXT

Version History

• Revision 1, 2019-03-05 (Neil Henning)

  • Initial draft

• Revision 2, 2019-07-26 (Jason Ekstrand)

  • Add the missing PhysicalDeviceSubgroupSizeControlFeaturesEXT for querying subgroup size control features.

See Also

PhysicalDeviceSubgroupSizeControlFeaturesEXT, PhysicalDeviceSubgroupSizeControlPropertiesEXT, PipelineShaderStageRequiredSubgroupSizeCreateInfoEXT

Document Notes

For more information, see the Vulkan Specification

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