Synchronization Validation
Synchronization Validation is implemented in the VK_LAYER_KHRONOS_validation layer
. When enabled, the Synchronization Object is intended to identify resource access conflicts due to missing or incorrect synchronization operations between actions (Draw, Copy, Dispatch, Blit) reading or writing the same regions of memory.
Synchronization will ideally be run periodically after resolving any outstanding validation checks from all other validation objects, so that issues may be addressed in early stages of development.
Synchronization can easily be enabled and configured using the Vulkan Configurator included with the Vulkan SDK. You can also manually enable Synchronization following instructions below.
The specific areas covered by this layer are currently tracked in the Synchronization Validation Project. Requests for additional checks can be requested by creating a Github issue.
The pipelined and multi-threaded nature of Vulkan makes it particularly important for applications to correctly insert needed synchronization primitives, and for validation to diagnose unprotected memory access hazards. Synchronization reports the presence of access hazards including information to identify the Vulkan operations which are in conflict. The reported hazards are:
VK_KHR_synchronization2
extensionSynchronization Validation is disabled by default. To turn on Synchronization Validation, add the following to your layer settings file, vk_layer_settings.txt
:
khronos_validation.enables = VK_VALIDATION_FEATURE_ENABLE_SYNCHRONIZATION_VALIDATION_EXT
To enable using environment variables, set the following variable:
VK_LAYER_ENABLES=VK_VALIDATION_FEATURE_ENABLE_SYNCHRONIZATION_VALIDATION_EXT
Mobile platforms (such as Android) do not support configuration of the validation layers with this configuration file. Programs running on these platforms must then use the programmatic interface.
As Synchronization Validation is resource intensive, it is recommended to disable all other validation layer objects.
The VK_EXT_validation_features
extension can be used to enable Synchronization Validation at CreateInstance time.
Here is sample code illustrating how to enable it:
VkValidationFeatureEnableEXT enables[] = {VK_VALIDATION_FEATURE_ENABLE_SYNCHRONIZATION_VALIDATION_EXT}; VkValidationFeaturesEXT features = {}; features.sType = VK_STRUCTURE_TYPE_VALIDATION_FEATURES_EXT; features.enabledValidationFeatureCount = 1; features.pEnabledValidationFeatures = enables; VkInstanceCreateInfo info = {}; info.pNext = &features;
To debug synchronization validation issues (all platforms):
vkCreateDebugUtilsMessengerEXT
with the VK_DEBUG_REPORT_ERROR_BIT_EXT
set.vkCmd
... command with a hazard is recorded.On Windows, Synchronization Validation can be run using just vkconfig and the debugger without defining a callback:
vkCmd
... command with a hazard is recorded.All synchronization error messages begin with SYNC-<hazard name>. The message body is constructed:
<cmd name>: Hazard <hazard name> <command specific details> Access info (<...>)
Command specific details typically include the specifics of the access within the current command. The Access info is common to all Synchronization Validation error messages.
VK_SUBPASS_EXTERNAL
when memory barriers are needed.Access info read_barrier
and write_barrier
values of 0, reflect the absence of any barrier, and can indicate an insufficient or incorrect source mask (first scope).VK_PIPELINE_STAGE_ALL_COMMANDS_BIT
, VK_ACCESS_MEMORY_READ_BIT
|VK_ACCESS_MEMORY_WRITE_BIT
for both src*Mask
and dst*Mask
fields to locate missing barriers. If the inserted barrier resolves a hazard, the conflicting access happens-before the inserted barrier. (Be sure to delete later.)Synchronization Examples https://github.com/KhronosGroup/Vulkan-Docs/wiki/Synchronization-Examples
Keeping your GPU fed without getting bitten https://www.youtube.com/watch?v=oF7vOTTaAh4
Yet another blog explaining Vulkan synchronization http://themaister.net/blog/2019/08/14/yet-another-blog-explaining-vulkan-synchronization/
A Guide to Vulkan Synchronization Validation https://www.khronos.org/news/permalink/blog-a-guide-to-vulkan-synchronization-validation