Vulkan: Architecture positive How Vulkan maps to PowerVR GPUs Kevin sun Lead Developer Support Engineer, APAC PowerVR Graphics.

Transcription

1 Vulkan: Architecture positive How Vulkan maps to PowerVR GPUs Kevin sun Lead Developer Support Engineer, APAC PowerVR Graphics

2 Introduction Who am I? Kevin Sun Working at Imagination Technologies Take responsibility for PowerVR technical support in APAC region Imagination Technologies Vulkan: High efficiency on mobile

3 Introduction This series Detailed look at Vulkan No new information about the API Khronos job! Adding context to existing information Primarily from SIGGRAPH This week: How Vulkan maps to PowerVR GPUs Imagination Technologies Vulkan: High efficiency on mobile

4 OpenGL

5 OpenGL OpenGL is over 23 years old now That s a long legacy Hardware has moved on! Imagination Technologies Vulkan: High efficiency on mobile

6 OpenGL State Mapping State doesn t map well to hardware Many functions with no indication of cost Modern hardware is complex Some state changes can cause pipeline bubbles Some state not really state May be expressed as shader code Requires patching into a shader, after compilation Imagination Technologies Vulkan: High efficiency on mobile

7 OpenGL Implicit Synchronization Many operations assume synchronization Can lead to excessive cache flushing May force unrelated operations to be serialized Drivers have to try to work magic Heuristics all over the place Applications try to guess how to trigger them Imagination Technologies Vulkan: High efficiency on mobile

8 OpenGL Immediate Mode Commands execute in order Draw call is the unit of work Maps well enough to IMR architectures Though not as well in modern hardware Not a great fit for Tile-Based architectures Unit of work is a render pass, sort of Imagination Technologies Vulkan: High efficiency on mobile

9 OpenGL Immediate Mode vs. TBRs Two stages of rendering Tiling Sort geometry into tiles Rasterization rasterize each tile Rasterization of a tile stays on-chip Draw call mostly meaningless Flushing means splitting a render Load/store tile data from/to main memory Imagination Technologies Vulkan: High efficiency on mobile

10 Vulkan

11 Vulkan Pipelines + Static State Pipelines tie state to shader compilation Most state is considered static Some optionally dynamic state Goodbye shader patching State is baked in Hello pipeline caches and derivatives Results in many more objects Pipeline caches ensure no duplicate work Pipeline derivatives can be used to avoid excessive switching Imagination Technologies Vulkan: High efficiency on mobile

12 Vulkan Pipelines + Static State on PowerVR Blending is programmable Patched onto a shader at draw time Shader already optimised without blending Inefficiencies, though small, are introduced Having blend mode at compile time allows optimisation Other state falls into same category Framebuffer format Vertex layout Imagination Technologies Vulkan: High efficiency on mobile

13 Vulkan Explicit Synchronization More complex, but not excessively Not at the level of individual cache control Very expressive I need to access resource here I need this operation to have finished by this point Create optimal dependency chains Can avoid excessive flushing Imagination Technologies Vulkan: High efficiency on mobile

14 Vulkan Explicit Synchronization on PowerVR PowerVR is a TBDR So two stages of rendering All vertex work can run before starting rasterization Example: Rasterization depends on earlier task Geometry processing does not Little difference to an IMR PowerVR GPUs can keep working whilst earlier task completes Vulkan allows that dependency to be called out explicitly Imagination Technologies Vulkan: High efficiency on mobile

15 Vulkan Command Buffers + Queues Submission is deferred, not immediate Work is organised into chunks command buffers Implementations can batch work optimally Queues map to hardware queues Implementation exposes as many queues as exist Cannot submit work from multiple threads Imagination Technologies Vulkan: High efficiency on mobile

16 Vulkan Command Buffers + Queues on PowerVR Command buffer becomes a list of tasks Draws turn into tiling and rasterization tasks Dependencies are optimised Queue submission kicks these tasks Some implicit dependencies Rasterization waits on Tiling Some explicit dependencies Semaphores Imagination Technologies Vulkan: High efficiency on mobile

17 Vulkan Render passes Render passes group rendering commands Certain operations not permitted inside a render pass Avoids hidden mid-frame flushes Multiple sub-passes in a render pass Each can communicate via pixel-local image data Writes from one sub-pass can be visible in the next Sub-passes define load/store operations How rendering begins and ends for each sub-pass Imagination Technologies Vulkan: High efficiency on mobile

18 Vulkan Render passes on PowerVR (1/2) Useful to batch chunks of draws at a time Allows rasterization of each to remain on-chip Flushes require loading and storing image data Costs bandwidth, so want to avoid this Sub-pass defined so it can be kept on-chip Load/Store define transfers to/from on-chip memory Keeps bandwidth low Imagination Technologies Vulkan: High efficiency on mobile

19 Vulkan Render passes on PowerVR (2/2) Render pass is a collection of sub-passes Sub-passes may be combined Communication between sub-passes is pixel-local EXT_shader_pixel_local_storage Input attachments express on-chip registers Expresses framebuffer_fetch/pls equivalent in core Allows TBRs to do what they do best Attachments don t need to be allocated in many cases Imagination Technologies Vulkan: High efficiency on mobile

20 Conclusion

21 Conclusion Vulkan designed to map to modern HW Requires a lot more information from an application Maps very well to PowerVR GPUs Should map equally well to other hardware Render Passes in particular very useful! Consistent performance across architectures No hidden expensive operations No need for driver heroics to avoid them Imagination Technologies Vulkan: High efficiency on mobile

22 Vulkan: Architecture positive How Vulkan maps to PowerVR GPUs Kevin sun Lead Developer Support Engineer, APAC PowerVR Graphics