Registered Extension Number

226

Revision

2

Extension and Version Dependencies

  • Requires Vulkan 1.1

Other Extension Metadata

Last Modified Date

2019-03-05

Contributors
  • Jeff Bolz, NVIDIA

  • Jason Ekstrand, Intel

  • SÅ‚awek 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 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:

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

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.

New Enum Constants

  • Extending VkStructureType:

    • VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_SUBGROUP_SIZE_CONTROL_FEATURES_EXT (added in version 2)

    • VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_SUBGROUP_SIZE_CONTROL_PROPERTIES_EXT

    • VK_STRUCTURE_TYPE_PIPELINE_SHADER_STAGE_REQUIRED_SUBGROUP_SIZE_CREATE_INFO_EXT

  • Extending VkPipelineShaderStageCreateFlagBits:

    • VK_PIPELINE_SHADER_STAGE_CREATE_ALLOW_VARYING_SUBGROUP_SIZE_BIT_EXT

    • VK_PIPELINE_SHADER_STAGE_CREATE_REQUIRE_FULL_SUBGROUPS_BIT_EXT

Issues

None.

Version History

See Also

Document Notes

For more information, see the Vulkan Specification

This page is a generated document. Fixes and changes should be made to the generator scripts, not directly.

Copyright (c) 2014-2020 Khronos Group. This work is licensed under a Creative Commons Attribution 4.0 International License.