A Single (Unified) Shader GPU Microarchitecture for Embedded Systems

1. A Single (Unified) Shader GPU Microarchitecture for Embedded Systems Victor Moya, Carlos González, Jordi Roca, Agustín Fernández Department of Computer Architecture UPC Roger Espasa Intel DEG Barcelona

2. Introduction • Graphics and specifically 3D graphics have become an important element in current PDA, mobile phone and other handheld systems • OpenGL ES: A simplified OpenGL specification for embedded systems • The classic GPU architecture for the PC is not suited for embedded systems • Low power • Low area budget • We propose a single unified shader GPU architecture for embedded systems

3. Outline • ATTILA PC • ATTILA Embedded • Triangle Setup in the Shader Unit • ATTILA Simulation Framework • Results

4. Outline • ATTILA PC • ATTILA Embedded • Triangle Setup in the Shader Unit • ATTILA Simulation Framework • Results

5. Attila Classic for PCs • Optimized for large resolutions • Above 1024x768 • Optimized for high performance • High power requirements • No power optimizations • 100+ watts on current high-end GPUs • Large area budget • 300+ million transistors on current high-end GPUs • Large dedicated of memory bandwidth • 40+ GB/s on current high-end GPUs • Specialized Shader Units • 2 to 8 vertex shader units • 1 to 6 fragment shader units

6. Vertex Fetch Attila PC Vertex Shader Vertex Shader Primitive Assembly Clipping Specialized Shaders Triangle Setup Rasterization HierarchicalZ Fragment Shader Fragment Shader Four fragments processed in parallel ROP ROP Memory Controller Memory Controller

7. Outline • ATTILA PC • ATTILA Embedded • Triangle Setup in the Shader Unit • ATTILA Simulation Framework • Results

8. Embedded Requirements • Optimized for small resolutions • 320x240 to 640x480 • Optimized for low power • Reduce frequency • Power optimizations • Improve efficiency • Small area budget • Remove non crucial hardware • Low available bandwidth • Reduced shading power • Reduce design complexity

9. Attila Embedded • No Hierarchical Z • No Z compression • Single unified shader • 1 SIMD ALU • Multithreaded • 16 threads of four vertex/triangle/fragment elements • 16 128-bit registers for temporal storage available per thread • Texture unit outputs 1 bilinear for a whole fragment quad each 4 cycles • 4 KB Texture Cache • ROP • One z and one color values updated per cycle in the framebuffer (a fragment quad each 4 cycles). • Single 64-bit DDR channel • Limited by current simulator implementation • Assimilated to small (1 MB) embedded DRAM • 32-bit high latency bus to large system memory for textures

10. Attila Embedded Vertex Fetch Single Unified Shader Primitive Assembly Scheduler Distributor Shader Clipping Rasterization Memory Controller ROP Single fragment per cycle pipeline Vertices Triangles Fragments

11. Outline • ATTILA PC • ATTILA Embedded • Triangle Setup in the Shader Unit • ATTILA Simulation Framework • Results

12. Triangle Setup in the Shader • 2D Homogeneous Rasterization • Olano & Greer • Triangle setup algorithm: • Calculate setup matrix from triangle vertex matrix • Calculate interpolation equation for fragment Z • Cull triangles based on their facing direction (area sign) • Algorithm suited for a SIMD implementation in the Unified Shader • Inputs: • Four 3 component vectors as input for the triangle vertex positions • Outputs: • Three 4 component vectors as output for the triangle edge and z interpolation equation coefficients. • One signed triangle area register as output for face culling stage • 26 Instruction Triangle Shader program

13. Triangle Setup in the Shader • Benefits • Reduce area • No specialized hardware required for Triangle setup • Reduce design complexity • Improve efficiency • Graphic workload in embedded applications may not fully utilize the triangle setup specialized hardware in most cases • Higher utilization of the shader • Costs • Shader workload increases • Rerouting of the rasterization pipeline required

14. Outline • ATTILA PC • ATTILA Embedded • Triangle Setup in the Shader Unit • ATTILA Simulation Framework • Results

15. Collect Verify Simulate Analyze OpenGL Application GLInterceptor Trace GLPlayer Statistics Vendor OpenGL Driver Vendor OpenGL Driver ATTILA OpenGL Driver Signal Traffic ATI R520/NVidia G70 ATI R520/NVidia G70 ATTILA Simulator Framebuffer Framebuffer Framebuffer Signal Visualizer CHECK! CHECK!

16. Collect Verify Simulate Analyze OpenGL Application • GLInterceptor • Capture a trace of OpenGL API calls from a real game GLInterceptor Trace GLPlayer Statistics Vendor OpenGL Driver Vendor OpenGL Driver ATTILA OpenGL Driver Signal Traffic ATI R520/NVidia G70 ATI R520/NVidia G70 ATTILA Simulator Framebuffer Framebuffer Framebuffer Signal Visualizer CHECK! CHECK!

17. Collect Verify Simulate Analyze OpenGL Application GLInterceptor • GLPlayer • Reproduce the captured trace Trace GLPlayer Statistics Vendor OpenGL Driver Vendor OpenGL Driver ATTILA OpenGL Driver Signal Traffic ATI R520/NVidia G70 ATI R520/NVidia G70 ATTILA Simulator Framebuffer Framebuffer Framebuffer Signal Visualizer CHECK! CHECK!

18. Collect Verify Simulate Analyze • OpenGL Library • - Transform Fixed Function API into Shader code • - 200 API calls supported • - ARB Vertex and Fragment extensions • - Alpha and Fog emulated via Shader code • Driver • - Low level interface to GPU hardware • - Attila memory management OpenGL Application GLInterceptor Trace GLPlayer Statistics Vendor OpenGL Driver Vendor OpenGL Driver ATTILA OpenGL Driver Signal Traffic ATI R520/NVidia G70 ATI R520/NVidia G70 ATTILA Simulator Framebuffer Framebuffer Framebuffer Signal Visualizer CHECK! CHECK!

19. Collect Verify Simulate Analyze • ATTILA Simulator • - Detailed cycle-by-cycle simulation of all pipeline stages • - 20 boxes, modeling a 100-deep pipeline • - Execute@Execute: functionality embedded at each pipeline stage OpenGL Application GLInterceptor Trace GLPlayer Statistics Vendor OpenGL Driver Vendor OpenGL Driver ATTILA OpenGL Driver Signal Traffic ATI R520/NVidia G70 ATI R520/NVidia G70 ATTILA Simulator Framebuffer Framebuffer Framebuffer Signal Visualizer CHECK! CHECK!

20. Spot the differences Attila NVidia GeForce FX 5900XT

21. Outline • ATTILA PC • ATTILA Embedded • Triangle Setup in the Shader Unit • ATTILA Simulation Framework • Results

22. Benchmark • Unreal Tournament 2004 • NOT AN EMBEDDED BENCHMARK • Up to 300K vertices per frame! • Fixed function OpenGL API • Vertex and fragments shaders generated by our library • 320x240 resolution • 140 of 450 frames simulated • 100+ frames ~ 1 day simulation • On a Xeon P4 @ 2.0Ghz

23. Configurations • We have evaluated • 3 middle-end to low-end PC GPU configurations • 2 integrated on chipset GPUs and high-end PDA GPUs configurations • 4 embedded low-end GPUs configurations • We tried to keep a balance between memory bandwidth and shading computing power • From 4 to no vertex shader units • From 2 quad fragment shader units to a single unified shader unit • From four to one 64-bit DDR memory channels • Store framebuffer in small (1 MB) GPU memory and textures in system memory • Halved the frequency for embedded systems • Restricted design rules • Reduce power consumption • Removed all optional features at the low end • Hierarchical Z • Z compression • Specialized Triangle Setup hardware

24. Evaluated Configurations

25. Configuration Comparison

26. Performance • Average of 20 frames per second at 320x240 for the lower end single shader configurations

27. Efficiency • The limiting factor for PC and high embedded configurations is memory bandwidth • Shaders underutilized for the evaluated benchmark • The limiting factor for low end configurations is shading processing • Memory bandwidth could be further reduced • Caches seem over dimensioned for the low-end embedded configurations

28. Shaded Triangle Setup Performance • No overhead on fragment limited benchmarks • 16% less performance in vertex and triangle limited traces

29. Conclusion • The Attila Embedded achieves 20 frames per second on a single unified shader architecture at a 320x240 resolution when using a year old PC benchmark • 1 MB of fast embedded DRAM provides more than enough bandwidth for framebuffer accesses • Texture data stored in system memory • 16% performance reduction when removing the specialized Triangle Setup unit in the worst tested case

30. Questions?

31. Attila PC Shader Vertex Fetch Shader Scheduler Distributor Primitive Assembly Clipping Shader Triangle Setup Rasterization Shader HierarchicalZ Unified Shader Pool ROP ROP ROP ROP Memory Controller Memory Controller Memory Controller Memory Controller

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