53 lines
3.2 KiB
Markdown
53 lines
3.2 KiB
Markdown
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# Fast OpenGL State Transitions
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Typical OpenGL programs issue a few small state change commands between draw call commands. We want
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the typical app's use case to be as fast as possible so this leads to unique performance challenges.
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Vulkan is quite different from OpenGL because it requires a separate compiled
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[VkPipeline][VkPipeline] for each state vector. Compiling VkPipelines is multiple orders of
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magnitude slower than enabling or disabling an OpenGL render state. To speed this up we use three
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levels of caching when transitioning states in the Vulkan back-end.
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## L3 Cache
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The outermost level is the driver's [VkPipelineCache][VkPipelineCache]. The driver
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cache reduces pipeline recompilation time significantly. But even cached
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pipeline recompilations are orders of manitude slower than OpenGL state changes.
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## L2 Cache
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The second level cache is an ANGLE-owned hash map from OpenGL state vectors to compiled pipelines.
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See [GraphicsPipelineCache][GraphicsPipelineCache] in [vk_cache_utils.h](../vk_cache_utils.h). ANGLE's
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[GraphicsPipelineDesc][GraphicsPipelineDesc] class is a tightly packed 256-byte description of the
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current OpenGL rendering state. We also use a [xxHash](https://github.com/Cyan4973/xxHash) for the
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fastest possible hash computation. The hash map speeds up state changes considerably. But it is
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still significantly slower than OpenGL implementations.
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## L1 Cache
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To get best performance we use a transition table from each OpenGL state vector to neighbouring
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state vectors. The transition table points from GraphicsPipelineCache entries directly to
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neighbouring VkPipeline objects. When the application changes state the state change bits are
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recorded into a compact bit mask that covers the GraphicsPipelineDesc state vector. Then on the next
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draw call we scan the transition bit mask and compare the GraphicsPipelineDesc of the current state
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vector and the state vector of the cached transition. With the hash map we compute a hash over the
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entire state vector and then do a 256-byte `memcmp` to guard against hash collisions. With the
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transition table we will only compare as many bytes as were changed in the transition bit mask. By
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skipping the expensive hashing and `memcmp` we can get as good or faster performance than native
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OpenGL drivers.
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Note that the current design of the transition table stores transitions in an unsorted list. If
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applications map from one state to many this will slow down the transition time. This could be
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improved in the future using a faster look up. For instance we could keep a sorted transition table
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or use a small hash map for transitions.
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## L0 Cache
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The current active PSO is stored as a handle in the `ContextVk` for use between draws with no state
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change.
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[VkPipeline]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkPipeline.html
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[VkPipelineCache]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkPipelineCache.html
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[GraphicsPipelineCache]: https://chromium.googlesource.com/angle/angle/+/225f08bf85a368f905362cdd1366e4795680452c/src/libANGLE/renderer/vulkan/vk_cache_utils.h#498
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[GraphicsPipelineDesc]: https://chromium.googlesource.com/angle/angle/+/225f08bf85a368f905362cdd1366e4795680452c/src/libANGLE/renderer/vulkan/vk_cache_utils.h#244
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