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VK_EXT_subgroup_size_control

Other Extension Metadata

Last Modified Date

2019-03-05

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

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≤SubgroupSize×maxComputeWorkgroupSubgroupss = { 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

New Enum Constants

  • VK_EXT_SUBGROUP_SIZE_CONTROL_EXTENSION_NAME
  • VK_EXT_SUBGROUP_SIZE_CONTROL_SPEC_VERSION
  • Extending VkPipelineShaderStageCreateFlagBits:
    • VK_PIPELINE_SHADER_STAGE_CREATE_ALLOW_VARYING_SUBGROUP_SIZE_BIT_EXT
    • VK_PIPELINE_SHADER_STAGE_CREATE_REQUIRE_FULL_SUBGROUPS_BIT_EXT
  • Extending VkStructureType:
    • VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_SUBGROUP_SIZE_CONTROL_FEATURES_EXT
    • VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_SUBGROUP_SIZE_CONTROL_PROPERTIES_EXT
    • VK_STRUCTURE_TYPE_PIPELINE_SHADER_STAGE_REQUIRED_SUBGROUP_SIZE_CREATE_INFO_EXT

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.

Version History