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GPU Memory Model Overview

GPU Memory Model Overview. Aaron Lefohn University of California, Davis. Overview. GPU Memory Model GPU-Based Data Structures Introduction to Frame Buffer Objects. Memory Hierarchy. CPU and GPU Memory Hierarchy. Disk. CPU Main Memory. CPU Caches. GPU Video Memory. CPU Registers.

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GPU Memory Model Overview

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  1. GPU Memory Model Overview Aaron Lefohn University of California, Davis

  2. Overview • GPU Memory Model • GPU-Based Data Structures • Introduction to Frame Buffer Objects

  3. Memory Hierarchy • CPU and GPU Memory Hierarchy Disk CPU Main Memory CPU Caches GPU Video Memory CPU Registers GPU Caches GPU Constant Registers GPU Temporary Registers

  4. CPU Memory Model • Random Memory Access at Any Program Point • Read/write to registers • Read/write to local (stack) memory • Read/write to global (heap) memory • Read/write to disk

  5. GPU Memory Model • Much more restricted memory access • GPU Kernels • Read/write to registers • No local stack memory • No disk access • Read-only global memory access v = a[i]; • Write to global memory at end of pass • Pre-computed memory addresses (no scatter) i = foo(a); a[i] = bar(b); • Write location set by fragment position a[fragPos] = bar(b);

  6. Vertex Processor Fragment Processor Rasterizer GPU Memory Model • Where is GPU Data Stored? • Vertex buffer • Texture • Frame buffer PS3.0 GPUs Texture Buffer Frame Buffer(s) Vertex Buffer

  7. Vertex Processor Fragment Processor Rasterizer Render-to-Texture • Idea • Write rendering result to texture memory • Enables GPU-based computational iterations PS3.0 GPUs Texture Data Vertex Data

  8. Render-to-Texture • OpenGL Support • Save up to 16, 32-bit floating values per pixel • Multiple Render Targets (MRTs) on ATI and NVIDIA • Copy-to-texture • glCopyTexSubImage • Render-to-texture • GL_EXT_framebuffer_object

  9. Vertex Processor Fragment Processor Rasterizer Render-to-Vertex-Array • Idea • Write rendering results to vertex array • Allows GPU to loop back to beginning of pipeline PS3.0 GPUs Texture Data Vertex Data

  10. Render-To-Vertex-Array • OpenGL Support • Copy-to-vertex-array • GL_ARB_pixel_buffer_object • NVIDIA and ATI • Render-to-vertex-array • Maybe someday as an extension to GL_EXT_framebuffer_object • Semantics Still Under Development…

  11. Vertex Processor Fragment Processor Rasterizer Fbuffer: Capturing Fragments • Idea • Save all fragment values instead of one per pixel • “Rasterization-Order FIFO Buffer” PS3.0 GPUs TextureData Frame Buffer(s) Vertex Data

  12. Fbuffer: Capturing Fragments • Details • Designed for multi-pass rendering with transparent geometry • Mark and Proudfoot, Graphics Hardware 2001 http://graphics.stanford.edu/projects/shading/pubs/hwws2001-fbuffer/ • New possibilities for GPGPU • Varying number of results per pixel • RTT and RTVA with an fbuffer • OpenGL Support • ATI Radeon 9800 and newer ATI GPUs • Still not exposed to user (ask for it!)

  13. Overview • GPU Memory Model • GPU-Based Data Structures • Introduction to Frame Buffer Objects

  14. GPU-Based Data Structures • Building Blocks • GPU memory addresses • Address Generation • Address Use • Pointers

  15. CPU Vertex Processor Fragment Processor Rasterizer GPU Memory Addresses • Where Are GPU Addresses Generated? • CPU Vertex stream or textures • Vertex processor Input stream, ALU ops or textures • Rasterizer Interpolation • Fragment processor Input stream, ALU ops or textures

  16. Fragment Processor Rasterizer CPU GPU Memory Addresses • Where Are Addresses Used? • Vertex textures (PS3.0 GPUs) • Fragment textures Texture Data Vertex Processor

  17. GPU Memory Addresses • Floating-Point Addressing • Normalized addresses [0,1] • GL_TEXTURE_1D, _2D, _3D, _CUBE • Non-Normalized addresses [0,N] • GL_TEXTURE_RECTANGLE • See Ian Buck’s warning about unaddressable texels

  18. 0 1 2 3 GPU Memory Addresses • Pointers • Store addresses in texture • Dependent texture read • Example: See Tim Purcell’s case study float2 addr = tex2D( addrTex, texCoord ); float2 data = tex2D( dataTex, addr ); Address Texture Data Texture 0 3 Data 1 3 Data 2 1 Data 1 3 Data

  19. GPU-Based Data Structures • Building Blocks • GPU memory addresses • Address Generation • Address Use • Pointers • Multi-dimensional arrays and structs • Sparse representations

  20. Easier GPU Data Structures? • This section describes the low-level details of implementing basic GPGPU data structures • See the “Glift” talk for a high-level view of GPU data structures • Glift is a GPU data structure template library • Now for the gory details…

  21. GPU Arrays • Large 1D Arrays • Current GPUs limit 1D array sizes to 2048 or 4096 • Pack into 2D memory • 1D-to-2D address translation

  22. GPU Arrays • 3D Arrays • Problem • GPUs do not have 3D frame buffers • No render-to-slice-of-3D-texture yet • Solutions • Stack of 2D slices • Multiple slices per 2D buffer

  23. GPU Arrays • Problems With 3D Arrays for GPGPU • Cannot read stack of 2D slices as 3D texture • Must know which slices are needed in advance • Visualization of 3D data difficult • Solutions • Flat 3D textures • Need render-to-slice-of-3D-texture • Maybe eventually with GL_EXT_framebuffer_object • Volume rendering of flattened 3D data • “Deferred Filtering: Rendering from Difficult Data Formats,” GPU Gems 2, Ch. 41, p. 667

  24. GPU Arrays • Higher Dimensional Arrays • Pack into 2D buffers • N-D to 2D address translation • Same problems as 3D arrays if data does not fit in a single 2D texture

  25. GPU Structures • Store each member in a different “array” (texture) • Update structs with Multiple-Render Targets (MRTs) struct Foo { struct Foo { float4 a; float4 a[N]; float4 b; float4 b[N]; }; }; Foo foo[N];

  26. GPU-Based Data Structures • Building Blocks • GPU memory addresses • Address Generation • Address Use • Pointers • Multi-dimensional arrays • Sparse representations

  27. Sparse Data Structures • Why Sparse Data Structures? • Reduce memory pressure • Reduce computational workload • Examples • Sparse matrices • Krueger et al., Siggraph 2003 • Bolz et al., Siggraph 2003 • Deformable implicit surfaces (sparse volumes/PDEs) • Lefohn et al., IEEE Visualization 2003 Premoze et al. Eurographics 2003

  28. Sparse Data Structures • Basic Idea • Pack “active” data elements into GPU memory • For more information • Linear algebra section in this course : Static structures • Adaptive grid case study in this course : Dynamic structures

  29. GPU Data Structures • Conclusions • Fundamental GPU memory primitive is a fixed-size 2D array • GPGPU needs more general memory model • Building and modifying complex GPU-based data structures is an open research topic…

  30. Overview • GPU Memory Model • GPU-Based Data Structures • Introduction to Frame Buffer Objects

  31. Introduction to Frame Buffer Objects • Where is the “Pbuffer Survival Guide”? • Gone!!! • Frame Buffer objects (FBO) recently replaced pbuffers • Frame Buffers enable simple, intuitive, fast render-to-texture in OpenGL • http://oss.sgi.com/projects/ogl-sample/registry/EXT/framebuffer_object.txt

  32. Frame Buffer Objects • What is an FBO? • A struct that holds pointers to memory objects • Each bound memory object can be a frame buffer rendering surface

  33. Frame Buffer Objects • Which memory can be bound to an FBO? • Textures • Render buffers • Depth, stencil, color • Traditional write-only frame buffer surfaces • Created with new API

  34. Frame Buffer Objects • FBO Tips and Tricks • Driver support still evolving • Try test in GPUBench to test your driver/GPU • Do not bind textures of different size or format to the same FBO • Attach/unattach textures is nearly as fast as glDrawBuffer if format/size is the same • Changing FBOs to use a different format/size is slower than keeping same format, but still faster than wglMakeCurrent/glXMakeCurrent

  35. Conclusions • GPU Memory Model Evolving • Writable GPU memory forms loop-back in an otherwise feed-forward streaming pipeline • Memory model will continue to evolve as GPUs become more general stream processors • GPGPU Data Structures • Basic memory primitive is limited-size, 2D texture • Use address translation to fit all array dimensions into 2D • See “Glift” talk for GPU data structure template library

  36. Acknowledgements • Nick Triantos, Craig Kolb, Cass Everitt, Chris Seitz, NVIDIA • Mark Segal, Rob Mace, Arcot Preetham, Evan Hart, ATI • Brian Budge, Ph.D. student at UCDavis and NVIDIA intern • The other GPGPU Siggraph 2005 course presenters • John Owens, Ph.D. advisor, Univ. of California Davis • Ross Whitaker, M.S. advisor, SCI Institute, Univ. of Utah • National Science Foundation Graduate Fellowship • Pixar Animation Studios, summer internships

  37. Questions? • Thank you! • Google “Lefohn GPU” • http://graphics.cs.ucdavis.edu/~lefohn/

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