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DirectCompute: Capturing the Teraflop

PDC09-CL03. DirectCompute: Capturing the Teraflop. Chas. Boyd Architect Microsoft Corporation. Overview. Describing the GPU as a CPU Fundamental principles in familiar terms Problem Set Definition In what cases will I get the Teraflop? How to DirectCompute Step by Step Managing I/O

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DirectCompute: Capturing the Teraflop

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  1. PDC09-CL03 DirectCompute:Capturing the Teraflop Chas. Boyd Architect Microsoft Corporation

  2. Overview • Describing the GPU as a CPU • Fundamental principles in familiar terms • Problem Set Definition • In what cases will I get the Teraflop? • How to DirectCompute • Step by Step • Managing I/O • Most codes are I/O bound

  3. Current CPU 4 Cores 4 float wide SIMD 3GHz 48-96GFlops 2x HyperThreaded 64kB $L1/core 20GB/s to Memory $200 200W CPU 0 CPU 1 CPU 2 CPU 3 L2 Cache

  4. Current GPU 32 Cores 32 Float wide 1GHz 1TeraFlop 32x “HyperThreaded” 64kB $L1/Core 150GB/s to Mem $200, 200W SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD L2 Cache

  5. Comparison: Current Processors SIMD SIMD SIMD SIMD SIMD CPU 0 CPU 1 SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD CPU 2 CPU 3 SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD SIMD L2 Cache SIMD SIMD SIMD SIMD SIMD L2 Cache CPU GPU

  6. CPU vs GPU CPU GPU High bandwidth memory Sequential accesses 100GB/s bandwidth 1TFlop compute 10 Gflops/watt Niche programming model • Low latency memory • Random accesses • 20GB/s bandwidth • 0.1TFlop compute • 1GFlops/watt • Well known programming model

  7. An Asymmetric Multi- Processor System CPU 50GFlops GPU 1TFlop 1GB/s 10GB/s 100GB/s CPU RAM • 4-6 GB GPU RAM • 1 GB

  8. GPUs are Data-Parallel Processors • GPU has 1000s of simultaneous ALUs • Need 100s of 1000s of threads to hit peak • Only data elements come in such numbers

  9. GPUs Need Data-Parallel Algorithms • Image processing • Reduction, Histogram, FFT, Summed Area Table • Video processing • transcode, effects, analysis • Audio • Linear Algebra • Simulation/Modeling: • Technical, Finance, Academic • Some Databases

  10. Video Stabilization video

  11. Applications <> Algorithms • Most important algorithms have known data-parallel versions • Algorithm was replaced with data-parallel version: • Sorting: Quicksort was swapped to Bitonic

  12. N-Body Galaxy Simulation demo DirectCompute AMD HD 5870 DirectX11

  13. The Teraflop Today N-Body Demo App: AMD Phenom II X4 940 3GHz + Radeon HD 5850 CPU      13.7GFlops Multicore SSE, not cache-aware GPU   537GFlops DirectCompute Intel Xeon E5410 2.33GHz + Radeon HD 5870 CPU 25.5GFlops Multicore SSE, not cache-aware GPU   722GFlops DirectCompute

  14. After

  15. Microsoft FFT Performance GFlops Log2( size)

  16. Component Relationships Applications Media playback or processing, media UI, recognition, etc. Technical Domain Libraries Domain Languages Accelerator, Brook+, Rapidmind, Ct MKL, ACML, cuFFT, D3DX, etc. Compute Languages DirectCompute, CUDA, CAL, OpenCL, LRB Native, etc. Processors CPU, GPU, Larrabee nVidia, Intel, AMD, S3, etc.

  17. DirectCompute Adds Client Scenarios • Support for multiple vendors • All DirectX11 chips will support DirectCompute • Some DirectX10 chips already support it • Tight integration with rendering • Client scenarios involve interactive playback • Support media data-types • Hardware format conversion for pixel formats • Server scenarios still supported

  18. Code Walkthrough

  19. DirectCompute Usage • Initialize DirectCompute • Create some GPU code in .hlsl • Compile it using DirectX compiler • Load the code onto the GPU • Set up a GPU buffer for input data • And set up a view into it for access • Make that data view current • Execute the code on the GPU • Copy the data back to CPU memory

  20. Initialize DirectCompute • hr = D3D11CreateDevice( • NULL, // default gfx adapter • D3D_DRIVER_TYPE_HARDWARE, // use hw • NULL, // not swrasterizer • uCreationFlags, // Debug, Threaded, etc. • NULL, // feature levels • 0, // size of above • D3D11_SDK_VERSION, // SDK version • ppDeviceOut, // D3D Device • &FeatureLevelOut, // of actual device • ppContextOut ); // subunit of device • );

  21. Example HLSL code • #define BLOCK_SIZE 256 • StructuredBuffer gBuf1; • StructuredBuffer gBuf2; • RWStructuredBuffergBufOut; • [numthreads(BLOCK_SIZE,1,1)] • void VectorAdd( uint3 id: SV_DispatchThreadID ) • { • gBufOut[id] = gBuf1[id] + gBuf2[id]; • }

  22. HLSL is the most widely used language for Data Parallel Programming Syntax is similar to ‘C/C++’ Preprocessor defines (#define, #ifdef, etc) Basic types (float, int, uint, bool, etc) Operators, variables, functions Has some important differences No pointers  Built-in variables & types (float4, matrix, etc) Intrinsic functions (mul, normalize, etc) The HLSL Language

  23. Compile the HLSL code • hr = D3DX11CompileFromFile( • “myCode.hlsl”, // path to .hlsl file • NULL, • NULL, • “VectorAdd”, // entry point • pProfile, • NULL, // Flags • NULL, • NULL, • &pBlob, // compiled shader • &pErrorBlob, // error log • NULL );

  24. Compiler (fxcor library) generates target-specific instructions (IL) from shader Different instruction sets for different generations of hardware Shader IL is highly optimized Compilation Steps

  25. Complete Compilation and Send to GPU • pD3D->CreateComputeShader( • pBlob->GetBufferPointer(), • pBlob->GetBufferSize(), • NULL, • &pMyShader ); // hw fmt • pD3D->CSSetShader( • pMyShader, NULL, 0 );

  26. Setup Buffer Resource for Input Data • D3D11_BUFFER_DESC descBuf; • ZeroMemory( &descBuf, sizeof(descBuf) ); • desc.BindFlags = D3D11_BIND_UNORDERED_ACCESS; • desc.StructureByteStride = uElementSize; • desc.ByteWidth = uElementSize * uCount; • desc.MiscFlags = • D3D11_RESOURCE_MISC_BUFFER_STRUCTURED; • pD3D->CreateBuffer( &desc, pInput, ppBuffer );

  27. Resources • Resource Objects are used to store data • Resource Views are interfaces to the Resource Compute Shader Sampler Resource View Resource Object My Data Buffer Unordered Access View

  28. DirectX Resources • Data Objects in memory • Enable out-of-bounds memory checking • Improves security, reliability of shipped code • Returns 0 on reads • Writes are No-Ops • Facilitates interop with Direct3D for display

  29. DirectX Resource Types • Buffer • Defines an arbitrary data struct for the records in this buffer object • Includes, structured, raw, streaming buffers • Texture* • Storage for data that will be used in pixel tasks • Includes 1-D, 2-D, 3-D, Cubes and arrays thereof

  30. Buffer Resource Types • Structured • Defines a record size with a fixed size. • Pixel data format is not specified, so automatic type/format conversion not provided • Unstructured • Can provide type/format conversion • Both types support non-order-preserving • For use with Append()/Consume() I/O

  31. Image/Media Resource Types • Texture1D, 2D, 3D, Cube, Array • A 2-D array of Pixels in specified format • R8G8B8A8, R32_UINT, R16G16_UINT

  32. Setup a View into the Buffer • D3D11_UNORDERED_ACCESS_VIEW_DESC desc; • ZeroMemory( &desc, sizeof(desc) ); • desc.ViewDimension = D3D11_UAV_DIMENSION_BUFFER; • desc.Buffer.FirstElement = 0; • desc.Format = DXGI_FORMAT_UNKNOWN; • desc.Buffer.NumElements = uCount; • pD3D->CreateUnorderedAccessView( • pBuffer, // Buffer view is into • &desc, // above data • &pMyUAV ); // result

  33. Resource Views • Resource Views define the access mechanism for data stored in Resources (buffers) • Support cool features like: • Hardware accelerated format conversion • Hardware accelerated linear filtering/sampling • Can create multiple views onto one resource • Enable data polymorphism while providing info to implementation for optimal layout

  34. Unordered Access View (UAV) • Enables two alternative usage patterns: • Unordered/random/scattered I/O to the buffer it is created into • Indexed operations for I/O • myBuffer[index] = x; • For Texture2D Resource, index is uint2 • Or Non-Order-Preserving I/O • Using Append()/Consume() intrinsics

  35. Non-Order Preserving I/O • For fastest performance when ordering of records need not be preserved • Or when nr of writes is unknown Append( ResourceVar, val); • Corresponding read operation provided for completeness Consume( ResourceVar, val); • Requires buffer to have flag enabling this

  36. Shader Resource View (SRV) • Enables hardware accelerated filtered sampling of the buffer • This hardware is a significant fraction of chip area • Excellent for pixel data (images/video) • A single pixel format defined per View • Read-Only operation • Same resource cannot be bound to shader as SRV and as another view type at the same time • Can also load w/o filtering

  37. Implementation Secrets • Resources correspond to ranges of memory • Views correspond to hardware logic units that perform data transformation on I/O

  38. Graphics vs Compute I/O Texture Samplers Pixel format conversion,Bi-linear filtering, Gamma correction ALUs Shader Execution Output Mergers Gamma correction, Pixel format conversion, Framebufferprefetch ~50 clocks 250 clocks GPU Memory

  39. Bind the Data, Launch the Work • pD3D->CSSetUnorderedAccessViews( • 0, • 1, • &pMyUAV, • NULL ); • pD3D->Dispatch( GrpsX, GrpsY, GrpsZ );

  40. Thread Groups • Not all threads in the call can/should share registers with each other • Compute threads are structured into subsets or groups of threads • Thread indices are available to the code: • SV_DispatchThreadID index of thread in call • SV_GroupThreadID index of thread in group • SV_GroupID index of group in call

  41. Thread Groups • pDev11->Dispatch(3, 2, 1); • [numthreads(4, 4, 1)] • void MyCS(…) 00 00 01 02 03 10 00 01 02 03 20 00 01 02 03 10 11 12 13 10 11 12 13 10 11 12 13 20 21 22 23 20 21 22 23 20 21 22 23 30 31 32 33 30 31 32 33 30 31 32 33 01 00 01 02 03 11 00 01 02 03 00 21 01 02 03 10 11 12 13 10 11 12 13 10 11 12 13 20 21 22 23 20 21 22 23 20 21 22 23 30 31 32 33 30 31 32 33 30 31 32 33

  42. Set up Buffer for Transfer to CPU • D3D11_BUFFER_DESC desc; • ZeroMemory( &desc, sizeof(desc) ); • desc.CPUAccessFlags = • D3D11_CPU_ACCESS_READ; • desc.Usage = D3D11_USAGE_STAGING; • desc.BindFlags = 0; • desc.MiscFlags = 0; • pD3D->CreateBuffer( • &desc, NULL, &StagingBuf );

  43. Transfer Results to CPU • pD3D->CopyResource( debugbuf, pBuffer );

  44. Temporary Registersaka General Purpose Registers • Used for fast local variable storage • Built as a block in each SIMD core • 16k 32-bit registers per core • Registers available per thread depends on number of threads in the group (group size) • E.g. 16k registers/1024 threads in group means each thread gets 16 DWORDs • Exceeding this limit has perf impacts: • Registers may be spilled to memory, or • Threads on core may be cut back (less ‘HyperThreads’)

  45. Groupshared Memory • New register type variable storage class • groupshared float sfFoo; • A whole group of threads can access the same memory • Enables uses like user-controlled cache • Max 32kB can be shared in DirectX11 • 8k floats or 2k float4s • Vs 64kB of temporary registers • 16k floats or 4k float4s • Using fewer is usually faster

  46. Barrier Intrinsics GroupMemoryBarrier DeviceMemoryBarrier AllMemoryBarrier • All I/O ops at the specified scope (group, device, or both) before this point must complete before any other I/O ops GroupMemoryBarrierWithGroupSync DeviceMemoryBarrierWithGroupSync AllMemoryBarrierWithGroupSync • All I/O ops at the specified scope (group, device, or both) before this point must complete before any other I/O ops • AND all the specified threads must reach this point before any can continue

  47. Barrier Example Shader() { groupshared GS[GROUPSIZE]; …compute the indices… GS[sid] = myBuffer[Tid]; // Load my data element GroupMemoryBarrierWithGroupSync(); // process the data in groupshared memory … … GroupMemoryBarrierWithGroupSync(); outBuffer[Tid] = GS[sid]; // write my data element }

  48. Implementation Secrets • Thread Group corresponds to a SIMD core • 1 of 16-32 on the die • Groupshared memory corresponds to a partition of that core’s L1 cache • GroupMemoryBarrier() corresponds to a flush of that core’s I/O

  49. Data Parallel I/O • I/O with 1600 active threads is not trivial • Reads are broadcast, so should be fast, but: • Writes by many threads to one destination can result in serialization • Less Obvious: • Even writing to a sequential location results in serialization on access to the address counter • This is why DirectCompute provides a rich set of I/O operations and intrinsics

  50. Hardware Support • DirectX11 Compute Shader runs on most current DirectX10 and 10.1 (4.x) parts • Explicit thread Dispatch() • Random-access I/O via resource variables • Private Write/Shared Read on groupshared data • New DirectX11-class (5.x) hardware adds • Arbitrary accesses to groupshared data • Atomic intrinsic operators • Hardware format conversion on i/o • More streaming i/o methods

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