Vulkan Pipeline Cache Staleness Detection: Defining Responsibility for Invalidating Stale Blobs

The application should treat a mismatch in VkPhysicalDeviceProperties::pipelineCacheUUID as a sufficient cause to discard its persisted cache. While the Vulkan specification requires drivers to ignore or reject incompatible blobs, application-side invalidation prevents wasting I/O and memory on data the driver will inevitably discard.

Implementation Responsibility

Responsibility for staleness detection is shared, but serves different purposes:

• The Driver: Ensures correctness. It validates the cache header (Vendor ID, Device ID, and UUID) during vkCreatePipelineCache. If the blob is incompatible, the driver will ignore the data, effectively treating it as an empty cache.
• The Application: Ensures efficiency. By tracking the pipelineCacheUUID, the application avoids loading large, useless blobs from disk and can proactively manage storage.

Analysis of Detection Methods

Using FAIL_ON_PIPELINE_COMPILE_REQUIRED_BIT

This bit (introduced in Vulkan 1.3 or via VK_EXT_pipeline_creation_cache_control) cannot be relied upon as a universal staleness detector. It indicates whether a pipeline must be compiled, but it does not specify why. A failure could be caused by a stale cache, but it could also be caused by a missing entry for a specific pipeline state that was never cached. Furthermore, this feature must be explicitly enabled and is not supported by all legacy implementations.

Recommended Versioning Strategy

To minimize unnecessary recompilation while ensuring stability, use a composite key for your persisted cache files. Do not rely on the UUID alone, as some drivers may not update the UUID for minor patches that still maintain binary compatibility, or conversely, may change it for updates that could have been compatible.

Recommended Key Components:

VkPhysicalDeviceProperties::pipelineCacheUUID: The primary hardware/driver compatibility signal.

Driver Version String: To catch updates where the UUID might remain static but the internal format changes.

Application Schema Version: A version number for your own shader layouts or pipeline state definitions; if you change a shader, the cache is stale regardless of the driver.

Verification Steps

To verify your invalidation logic, perform the following:

1. Log the current pipelineCacheUUID and driver version at startup and compare them against the metadata stored with your cache file.
2. Measure the time for vkCreateGraphicsPipelines with a warm cache versus a deliberately invalidated cache to confirm the driver is actually utilizing the blob.
3. Query VkPhysicalDeviceProperties to ensure pipelineCreationCacheControl is supported before attempting to use FAIL_ON_PIPELINE_COMPILE_REQUIRED_BIT.