VK_EXT_subpass_merge_feedback
1. Problem Statement
The Vulkan render pass mechanism allows the implementation to merge subpasses at render pass creation time to improve performance or reduce memory bandwidth usage. However, the criteria for merging are implementation-specific and not visible to applications, and the application has no way to know which (if any) subpasses were merged.
For performance debugging, applications may want to query whether certain subpasses were merged, or why they were not. They may also want to disable subpass merging completely or for certain subpasses, in order to determine the impact of subpass merging on performance.
This proposal attempts to provide feedback and control for the subpass merging. It can be used for performance debugging or for runtime usage.
2. Solution Space
Some new structures can be chained to pNext of VkRenderPassCreateInfo2, then the driver could provide subpass merging feedback and control subpass merging at render pass creation time.
If the subpasses cannot be merged, the reason could be returned. The driver could change the conditions of subpass merging based on performance result, or being compatible with future new extensions, so using a string to store the reason is more flexible than an enumeration.
3. Proposal
The following feature is exposed by the VK_EXT_subpass_merge_feedback extension:
typedef struct VkPhysicalDeviceSubpassMergeFeedbackFeaturesEXT {
VkStructureType sType;
void* pNext;
VkBool32 subpassMergeFeedback;
}subpassMergeFeedback is the main feature enabling this extension’s functionality and must
be supported if this extension is supported.
typedef struct VkRenderPassCreationControlEXT {
VkStructureType sType;
const void* pNext;
VkBool32 disallowMerging;
}The structure can control the subpass merging for both render pass level and subpass level. It can be chained to the pNext of VkRenderPassCreateInfo2 or VkSubpassDescription2.
typedef struct VkRenderPassCreationFeedbackCreateInfoEXT {
VkStructureType sType;
const void* pNext;
VkRenderPassCreationFeedbackInfoEXT* pRenderPassFeedback;
}The structure can get the feedback from render pass level at render pass creation time. It can be chained to the pNext of VkRenderPassCreateInfo2. The feedback information could tell application which subpasses are merged.
typedef struct VkRenderPassCreationFeedbackInfoEXT {
uint32_t postMergeSubpassCount;
}The structure contains the feedback data from render pass level.
typedef struct VkRenderPassSubpassFeedbackCreateInfoEXT {
VkStructureType sType;
const void* pNext;
VkRenderPassSubpassFeedbackInfoEXT* pSubpassFeedback;The structure can get the feedback from subpass level at render pass creation time. It can be chained to the pNext of VkSubpassDescription2. The feedback information could tell application whether a subpass is merged into previous subpass and what is the reason if the subpass cannot be merged.
typedef struct VkRenderPassSubpassFeedbackInfoEXT {
VkMergeStatusEXT mergeStatus;
char description[VK_MAX_DESCRIPTION_SIZE];
uint32_t postMergeIndex;
}The structure contains the feedback data from the subpass level.