VK_KHR_global_priority

Other Extension Metadata

Last Modified Date

2021-10-22

Contributors

• Tobias Hector, AMD
• Contributors to VK_EXT_global_priority
• Contributors to VK_EXT_global_priority_query

Description

In Vulkan, users can specify device-scope queue priorities. In some cases it may be useful to extend this concept to a system-wide scope. This device extension allows applications to query the global queue priorities supported by a queue family, and then set a priority when creating queues. The default queue priority is VK_QUEUE_GLOBAL_PRIORITY_MEDIUM_EXT.

Implementations can report which global priority levels are treated differently by the implementation. It is intended primarily for use in system integration along with certain platform-specific priority enforcement rules.

The driver implementation will attempt to skew hardware resource allocation in favor of the higher-priority task. Therefore, higher-priority work may retain similar latency and throughput characteristics even if the system is congested with lower priority work.

The global priority level of a queue shall take precedence over the per-process queue priority (VkDeviceQueueCreateInfo::pQueuePriorities).

Abuse of this feature may result in starving the rest of the system from hardware resources. Therefore, the driver implementation may deny requests to acquire a priority above the default priority (VK_QUEUE_GLOBAL_PRIORITY_MEDIUM_EXT) if the caller does not have sufficient privileges. In this scenario VK_ERROR_NOT_PERMITTED_EXT is returned.

The driver implementation may fail the queue allocation request if resources required to complete the operation have been exhausted (either by the same process or a different process). In this scenario VK_ERROR_INITIALIZATION_FAILED is returned.

New Structures

• Extending VkDeviceQueueCreateInfo:
  • VkDeviceQueueGlobalPriorityCreateInfoKHR
• Extending VkPhysicalDeviceFeatures2, VkDeviceCreateInfo:
  • VkPhysicalDeviceGlobalPriorityQueryFeaturesKHR
• Extending VkQueueFamilyProperties2:
  • VkQueueFamilyGlobalPriorityPropertiesKHR

New Enums

• VkQueueGlobalPriorityKHR

New Enum Constants

• VK_KHR_GLOBAL_PRIORITY_EXTENSION_NAME
• VK_KHR_GLOBAL_PRIORITY_SPEC_VERSION
• VK_MAX_GLOBAL_PRIORITY_SIZE_KHR
• Extending VkResult:
  • VK_ERROR_NOT_PERMITTED_KHR
• Extending VkStructureType:
  • VK_STRUCTURE_TYPE_DEVICE_QUEUE_GLOBAL_PRIORITY_CREATE_INFO_KHR
  • VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_GLOBAL_PRIORITY_QUERY_FEATURES_KHR
  • VK_STRUCTURE_TYPE_QUEUE_FAMILY_GLOBAL_PRIORITY_PROPERTIES_KHR

Issues

1) Can we additionally query whether a caller is permitted to acquire a specific global queue priority in this extension?

RESOLVED: No. Whether a caller has enough privilege goes with the OS, and the Vulkan driver cannot really guarantee that the privilege will not change in between this query and the actual queue creation call.

2) If more than 1 queue using global priority is requested, is there a good way to know which queue is failing the device creation?

RESOLVED: No. There is not a good way at this moment, and it is also not quite actionable for the applications to know that because the information may not be accurate. Queue creation can fail because of runtime constraints like insufficient privilege or lack of resource, and the failure is not necessarily tied to that particular queue configuration requested.

Version History

• Revision 1, 2021-10-22 (Tobias Hector)
  • Initial draft