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# Synchronization Validation

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](https://vulkan.lunarg.com/doc/sdk/latest/windows/vkconfig.html) 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](https://github.com/KhronosGroup/Vulkan-ValidationLayers/projects/5).
Requests for additional checks can be requested by creating a [Github issue](https://github.com/KhronosGroup/Vulkan-ValidationLayers/issues).

## Synchronization Validation Functionality

### Overview

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:


<table>
  <tr>
   <td>RAW
   </td>
   <td>Read-after-write
   </td>
   <td>Occurs when a subsequent operation uses the result of a previous operation without waiting for the result to be completed.
   </td>
  </tr>
  <tr>
   <td>WAR
   </td>
   <td>Write-after-read
   </td>
   <td>Occurs when a subsequent operation overwrites a memory location read by a previous operation before that operation is complete (requires only execution dependency).
   </td>
  </tr>
  <tr>
   <td>WAW
   </td>
   <td>Write-after-write
   </td>
   <td>Occurs when a subsequent operation writes to the same set of memory locations (in whole or in part) being written by a previous operation.
   </td>
  </tr>
  <tr>
   <td>WRW
   </td>
   <td>Write-racing-write
   </td>
   <td>Occurs when unsynchronized subpasses/queues perform writes to the same set of memory locations.
   </td>
  </tr>
  <tr>
   <td>RRW
   </td>
   <td>Read-racing-write
   </td>
   <td>Occurs when unsynchronized subpasses/queues perform read and write operations on the same set of memory locations.
   </td>
  </tr>
</table>



### Current Feature set

- Hazard detection for memory usage for commands within the *same* command buffer.
- Synchronization operations .
  - vkCmdPipelineBarrier.
  - vkCmdSetEvent/vkCmdWaitEvents/vkCmdResetEvent.
  - renderpass/subpass barriers.
- The `VK_KHR_synchronization2` extension
  - vkCmdPipelineBarrier2KHR
  - vkCmdSetEvent2KHR/vkCmdWaitEvents2KHR/vkCmdResetEvent2KHR.
- Image layout transition hazard and access tracking.
- Load/Store/Resolve operations within Subpasses.

### Known Limitations

- Does not include implementation of multi-view renderpass support.
- Host set event not supported.
- ExecuteCommands and QueueSubmit hazards from are not tracked or reported.
- Memory access checks not suppressed for VK_CULL_MODE_FRONT_AND_BACK.
- Does not include component granularity access tracking.
- Host synchronization not supported.

## Enabling Synchronization Validation

Synchronization Validation is disabled by default. To turn on Synchronization Validation, add the following to your layer settings file,
`vk_layer_settings.txt`:

```code
khronos_validation.enables = VK_VALIDATION_FEATURE_ENABLE_SYNCHRONIZATION_VALIDATION_EXT
```

To enable using environment variables, set the following variable:

```code
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.

### Enabling and Specifying Options with the Programmatic Interface

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:

```code
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;
```

## Typical Synchronization Validation Usage

### Debugging Synchronization Validation Issues

To debug synchronization validation issues (all platforms):

- Create a debug callback with `vkCreateDebugUtilsMessengerEXT` with the `VK_DEBUG_REPORT_ERROR_BIT_EXT` set.
- Enable synchronization as noted above. On Linux and Windows this can be simplified by enabling Synchronization Validation using [Vulkan Configurator (vkconfig)](https://vulkan.lunarg.com/doc/sdk/latest/windows/vkconfig.html).
- Set a breakpoint in the debug callback and run your application in the debugger.
- The callback will be called when a `vkCmd`... command with a hazard is recorded.

On Windows, Synchronization Validation can be run using just vkconfig and the debugger without defining a callback:

*   In vkconfig.
    *   Enable Synchronization Validation.
    *   Select 'Debug Actions' 'Break' and 'Debug Output'.
*   Debug application in Visual Studio.
*   Hazard messages will appear in the debugger output window and the debugger will break (in the validation layer code)  when a `vkCmd`... command with a hazard is recorded.


### Synchronization Validation Messages

All synchronization error messages begin with SYNC-&lt;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. 

<table>
  <tr>
   <td><strong>Field</strong>
   </td>
   <td><strong>Description</strong>
   </td>
  </tr>
  <tr>
   <td><code>usage</code>
   </td>
   <td>The stage/access of the current command
   </td>
  </tr>
  <tr>
   <td><code>prior_usage</code>
   </td>
   <td>The stage/access of the previous (hazarded) use
   </td>
  </tr>
  <tr>
   <td><code>read_barrier</code>
   </td>
   <td>For read <code>usage</code>, the list of stages with execution barriers between <code>prior_usage</code> and <code>usage</code>
   </td>
  </tr>
  <tr>
   <td><code>write_barrier</code>
   </td>
   <td>For write <code>usage</code>, the list of stage/access (in <code>usage</code> format) with memory barriers between <code>prior_usage</code> and <code>usage</code>
   </td>
  </tr>
  <tr>
   <td><code>command</code>
   </td>
   <td>The command that performed <code>prior_usage</code>
   </td>
  </tr>
  <tr>
   <td><code>seq_no</code>
   </td>
   <td>The zero based index of <code>command</code> within the command buffer
   </td>
  </tr>
  <tr>
   <td><code>reset_no</code>
   </td>
   <td>the reset count of the command buffer <code>command</code> is recorded to
   </td>
  </tr>
</table>


### Frequently Found Issues

*   Assuming Pipeline stages are logically extended with respect to memory access barriers.  Specifying the vertex shader stage in a barrier will **not** apply to all subsequent shader stages read/write access.
*   Invalid stage/access pairs (specifying a pipeline stage for which a given access is not valid) that yield no barrier.
*   Relying on implicit subpass dependencies with `VK_SUBPASS_EXTERNAL` when memory barriers are needed.
*   Missing memory dependencies with Image Layout Transitions from pipeline barrier or renderpass Begin/Next/End operations.
*   Missing stage/access scopes for load operations, noting that color and depth/stencil are done by different stage/access.


### Debugging Tips

*   Read and write barriers in the error message can help identify the synchronization operation (either subpass dependency or pipeline barrier) with insufficient or incorrect destination stage/access masks (second scope).
*   `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).
*   Insert additional barriers with stage/access `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 blogs/articles

Synchronization Examples[ https://github.com/KhronosGroup/Vulkan-Docs/wiki/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](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/](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

