VK_EXT_subgroup_size_control(3)

# VK_EXT_subgroup_size_control(3) Manual Page

## Name

VK_EXT_subgroup_size_control - device extension

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## 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

  - 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=W o r k G r oup S i z e.x×W o r k G r oup S i z e. y×W o r k g r oup S i z e. z≤S ub g r oup S i z e×max C ompute W o r k g r oup S ub g r oups

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 VkPhysicalDeviceFeatures2, VkDeviceCreateInfo:

  - VkPhysicalDeviceSubgroupSizeControlFeaturesEXT

- Extending VkPhysicalDeviceProperties2:

  - VkPhysicalDeviceSubgroupSizeControlPropertiesEXT

- Extending VkPipelineShaderStageCreateInfo:

  - VkPipelineShaderStageRequiredSubgroupSizeCreateInfoEXT

### 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`

### Version History

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

  - Initial draft

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

  - Add the missing VkPhysicalDeviceSubgroupSizeControlFeaturesEXT for querying subgroup size control features.

## See Also

VkPhysicalDeviceSubgroupSizeControlFeaturesEXT, VkPhysicalDeviceSubgroupSizeControlPropertiesEXT, VkPipelineShaderStageRequiredSubgroupSizeCreateInfoEXT

## 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

Copyright (c) 2014-2020 The Khronos Group Inc.

SPDX-License-Identifier: CC-BY-4.0