Parallelising Pipelined Wavefront Computations on the GPU

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1 Parallelising Pipelined Wavefront Computations on the GPU S.J. Pennycook G.R. Mudalige, S.D. Hammond, and S.A. Jarvis. High Performance Systems Group Department of Computer Science University of Warwick U.K 1st UK CUDA Developers Conference 7 th Dec 2009 Oxford, U.K. 1

2 Overview Wavefront Computations A GPGPU Solution? Wavefronts within Wavefronts Performance Modelling Beating the CPU Optimisations to Win Results, Validations and Model Projections Current and Future Work Conclusions 2

3 Wavefront Computations Wavefront computations are at the core of a number of large scientific computing workloads. Centers including the Los Alamos National Laboratory (LANL) in the United States and the Atomic Weapons Establishment (AWE) in the UK use these codes heavily. Lamport s core (hyperplane) algorithm that underpins these codes has existed for more than thirty five years. Defining characteristics: Operating on a grid of cells with each cell requiring some computation to be performed. Each cell has a data dependency, such that the solution of up to three neighbouring cells is required. 3

4 Cell Dependencies 4

5 Motivation Our previous work was on analysing and optimising applications that use the wavefront algorithm using MPI. Processor (1,m) Processor (1,1) Processor (n,m) Processor (n,1) Ny Nz Proceeds as Wavefronts through the 3D data cube Nx 5

6 Motivation (cont d) Algorithm operates over a three-dimensional structure of size Nx Ny Nz. Grid is mapped onto a 2D m x n grid of processors; each is assigned a stack of Nx / n x Ny / m x Nz cells. Data dependency results in a sequence of wavefronts (or a sweep) that starts from one corner and makes its way through other cells. We have modelled codes (e.g. Chimaera, LU, and Sweep3D) that employ wavefront computations with MPI. 6

7 Motivation (cont d) Our focus is now on using GPUs to investigate improvements to the solution per processor. A canonical solution is normally employed by the CPU to solve the computation per processor. Listing: Canonical Algorithm For k=1; k<=kend do For j=1; j<=jend do For i=1; i<=iend do A(i,j,k)=A(i 1,j,k)+ A(i,j 1,k)+A(i,j,k 1) // Compute cell End for End for End for 7

8 Hyperplane (Wavefront) Algorithm Let f = i + j + k, g = k and h = j. The plane defined by i + j + k = CONST is called a hyperplane. Listing : Hyperplane Algorithm DO CONCURRENTLY ON EACH PROCESSOR For f = 3, iend+jend+kend do A(f g h,h,g) = A(f g h 1,h,g)+A(f g h,h 1,g)+ A(f g h, h, g 1) End For The critical dependencies are preserved, even though the solution is carried out across the grid in wavefronts. 8

9 A GPGPU Solution? Can we utilise the many cores on a GPU to get a speedup to this algorithm? Theoretically simple... 9

10 A GPGPU Solution? (cont d) For a 3D cube of cells: 10

11 GPU Limitations What s the practical situation? Experimental System Daresbury Laboratory U.K. 8 x NVIDIA Tesla S1070 servers, each with four Tesla C1060 cards. Compute nodes consists of Nehalem processors (@ 2.53 GHz quad-core, 24 GB RAM). Each CPU core sees one Tesla card. Voltaire HCA410-4EX InfiniBand adapter. NVIDIA Tesla C1060 GPU Specifications: Each GPU card has 30 multi-processors Streaming Multiprocessor (SM) with 8 cores per processor. Each card therefore has 240 cores (streaming processor cores). Each core operates at to 1.44 GHz. 4 GB Memory per card. 11

12 GPU Limitations (cont d) CUDA Device Architecture: DRAM GPU To Host Local Global SM 1 SM 2 SM 4 Constant Texture Constant and Texture Cache Memory Registers SM 30 Shared Memory Processor Cores (8 cores) 12

13 GPU Limitations (cont d) Each SM is allocated a number of threads, arranged as blocks. No synchronisation between threads in different blocks. Limit of 512 threads per block. Memory hierarchy: Global memory access is slow and should be avoided. Limit of 16KB of shared memory per SM. Other considerations: Limit of 16,384 registers per block. Aligning half-warps for performance. 13

14 A Solution? Wavefronts within Wavefronts Need to be scalable. Run more than 512 threads by utilising parallelism across all the multiprocessors. The cells on each diagonal are decomposed into coarse subtasks, and assigned to an SM as thread blocks. 14

15 Wavefronts within Wavefronts Each diagonal is computed by a kernel: for (wave = 0; wave < (3*(N/dimBlock.x)) - 2; wave++) { // Run the kernel. hyperplane_3d <<< dimgrid, dimblock, shared_mem_size >>> (d_gpu, wave); } cudathreadsynchronize(); // Not strictly necessary. The time to compute one diagonal is ceiling (number of blocks per diagonal / number of SMs) Each block utilises the resources available to an SM to solve the cells we will talk about this later. 15

16 A Performance Model What does this solution mean in terms of a performance model? Modelling Block level performance Assume a 3D cube of data cells with dimension N P GPU Number of SMs on the GPU W g,gpu Time to solve a block of cells W GPU Time to solve the 3D cube of cells using the GPU 16

17 Initial Results Each cell is randomly initialised, and at each step calculates the average of itself and its top, north and west neighbours. How the 3D data is decomposed has a significant effect on execution time. Strange behaviour where the number of cells is a multiple of 32 (especially at powers of 2). 17

18 Initial Results (cont d) 18

19 Initial Results (cont d) 19

20 Initial Results (cont d) 20

21 Beating the CPU Optimisation within the blocks: Thread re-use. Caching values in shared memory. Coalesced memory accesses. Avoiding shared memory bank-conflicts. Optimisations over the blocks: Explicit vs implicit CPU synchronisation. Inter-block synchronisation using mutexes. 21

22 Thread Reuse in a Block Thread 0 Thread 1 Thread 2 Thread 3 Thread 4 Thread 5 Thread 6 Thread 7 Thread 8 Thread 9 Thread 10 Thread 11 Thread 12 Thread 13 Thread 14 Thread 15 22

23 Coalesced Memory Access Requires padding on devices below compute capability 1.3. How does this apply to 3D? 23

24 Beating the CPU (Results) 24

25 Beating the CPU (Results) Code was restructured for GPU to avoid unnecessary branching. Similar restructuring applied to CPU in kind. Re-use of threads and shared memory offers a 2x speedup over the naive GPU implementation. Spikes remain, likely to be an issue at the warp level. Kernel information: 17 registers bytes of shared memory per block. 42% occupancy. 25

26 The Bigger Picture Current work: Porting LU, Sweep3D and Chimaera to GPU. (CUDA and OpenCL) Additional barriers from larger programs: Double precision. Multiple computations per cell. Looking towards the future: How well does our algorithm perform on a consumer card (eg GTX 295)? How well will our algorithm perform on Fermi? Benchmarking and analysis should facilitate predictions. 26

27 Conclusions Wavefront computations can utilise emerging GPU architectures, despite their dependencies. To see speedup: Memcpy() needs to be faster. Require more work per Memcpy(). Codes cannot be ported naively. Hardware limitations may be a problem (particularly for larger codes). Performance modelling will offer insights into which applications can be ported successfully. 27

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