VK_KHR_timeline_semaphore(3) Manual Page

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This extension introduces a new type of semaphore that has an integer payload identifying a point in a timeline. Such timeline semaphores support the following operations:

All functionality in this extension is included in core Vulkan 1.2, with the KHR suffix omitted. The original type, enum and command names are still available as aliases of the core functionality.

1) Do we need a new object type for this?

RESOLVED: No, we just introduce a new type of semaphore object, as VK_KHR_external_semaphore_win32 already uses semaphores as the destination for importing D3D12 fence objects, which are semantically close/identical to the proposed synchronization primitive.

2) What type of payload the new synchronization primitive has?

RESOLVED: A 64-bit unsigned integer that can only be set to strictly increasing values by signal operations and is not changed by wait operations.

3) Does the new synchronization primitive have the same signal-before-wait requirement as the existing semaphores do?

RESOLVED: No. Timeline semaphores support signaling and waiting entirely asynchronously. It is the responsibility of the client to avoid deadlock.

4) Does the new synchronization primitive allow resetting its payload?

RESOLVED: No, allowing the payload value to “go backwards” is problematic. Applications looking for reset behavior should create a new instance of the synchronization primitive instead.

5) How do we enable host waits on the synchronization primitive?

RESOLVED: Both a non-blocking query of the current payload value of the synchronization primitive, and a blocking wait operation are provided.

6) How do we enable device waits and signals on the synchronization primitive?

RESOLVED: Similar to VK_KHR_external_semaphore_win32, this extension introduces a new structure that can be chained to VkSubmitInfo to specify the values signaled semaphores should be set to, and the values waited semaphores need to reach.

7) Can the new synchronization primitive be used to synchronize presentation and swapchain image acquisition operations?

RESOLVED: Some implementations may have problems with supporting that directly, thus it is not allowed in this extension.

8) Do we want to support external sharing of the new synchronization primitive type?

RESOLVED: Yes. Timeline semaphore specific external sharing capabilities can be queried using vkGetPhysicalDeviceExternalSemaphoreProperties by chaining the new VkSemaphoreTypeCreateInfoKHR structure to its pExternalSemaphoreInfo structure. This allows having a different set of external semaphore handle types supported for timeline semaphores vs. binary semaphores.

9) Do we need to add a host signal operation for the new synchronization primitive type?

RESOLVED: Yes. This helps in situations where one host thread submits a workload but another host thread has the information on when the workload is ready to be executed.

10) How should the new synchronization primitive interact with the ordering requirements of the original VkSemaphore?

RESOLVED: Prior to calling any command which may cause a wait operation on a binary semaphore, the client must ensure that the semaphore signal operation that has been submitted for execution and any semaphore signal operations on which it depends (if any) must have also been submitted for execution.

11) Should we have separate feature bits for different sub-features of timeline semaphores?

RESOLVED: No. The only feature which cannot be supported universally is timeline semaphore import/export. For import/export, the client is already required to query available external handle types via vkGetPhysicalDeviceExternalSemaphoreProperties and provide the semaphore type by adding a VkSemaphoreTypeCreateInfoKHR structure to the pNext chain of VkPhysicalDeviceExternalSemaphoreInfo so no new feature bit is required.

VkPhysicalDeviceTimelineSemaphoreFeaturesKHR, VkPhysicalDeviceTimelineSemaphorePropertiesKHR, VkSemaphoreSignalInfoKHR, VkSemaphoreTypeCreateInfoKHR, VkSemaphoreTypeKHR, VkSemaphoreWaitFlagBitsKHR, VkSemaphoreWaitFlagsKHR, VkSemaphoreWaitInfoKHR, VkTimelineSemaphoreSubmitInfoKHR, vkGetSemaphoreCounterValueKHR, vkSignalSemaphoreKHR, vkWaitSemaphoresKHR

For more information, see the Vulkan Specification

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