ME964 High Performance Computing for Engineering Applications CUDA Memory Model & CUDA API Sept. 16, 2008

Before we get started… Last Time Traced back the evolution of the GPU GPGPU and the CUDA step forward CUDA-related nomenclature Memory layout of typical NVIDIA GPU Today The CUDA API Start discussing CUDA programming model A look at a matrix multiplication example 2

The CUDA Access Situation You can install CUDA on your computer even if you don’t have a GPU card You can do 95% of your HW2 without needing a GPU Cards to be installed this afternoon in 1235ME CAE doesn’t want to made Visual Studio 2005 available (they use Visual Studio 2008) I’m looking into opening up my lab in case 1235ME doesn’t prove to be an option Linux accounts available at UIUC on GPU based supercomputer See Forum posting about details 3

After you unzip the ed assignment file, you should get a collection of files like below: HW2: A word on getting started with CUDA Double click helloworld.sln to get started The directory Linux contains a makefile and required files to get you going with this OS NOTE: readme.doc contains the text of the assignment 4

Execution Configuration: Grids and Blocks A kernel is executed as a grid of blocks of threads All threads share global memory space A block [of threads] is a batch of threads that can cooperate with each other by: Synchronizing their execution For hazard-free shared memory accesses Efficiently sharing data through a low latency shared memory Threads from two different blocks cannot cooperate!!! This has important software design implications Host Kernel 1 Kernel 2 Device Grid 1 Block (0, 0) Block (1, 0) Block (2, 0) Block (0, 1) Block (1, 1) Block (2, 1) Grid 2 Block (1, 1) Thread (0, 1) Thread (1, 1) Thread (2, 1) Thread (3, 1) Thread (4, 1) Thread (0, 2) Thread (1, 2) Thread (2, 2) Thread (3, 2) Thread (4, 2) Thread (0, 0) Thread (1, 0) Thread (2, 0) Thread (3, 0) Thread (4, 0) Courtesy: NDVIA 5 HK-UIUC

Block and Thread IDs Threads and blocks have IDs So each thread can decide what data to work on Block ID: 1D or 2D Thread ID: 1D, 2D, or 3D Why this 2D and 3D layout? Simplifies memory addressing when processing multidimensional data Image processing Solving PDEs on subdomains … Device Grid 1 Block (0, 0) Block (1, 0) Block (2, 0) Block (0, 1) Block (1, 1) Block (2, 1) Block (1, 1) Thread (0, 1) Thread (1, 1) Thread (2, 1) Thread (3, 1) Thread (4, 1) Thread (0, 2) Thread (1, 2) Thread (2, 2) Thread (3, 2) Thread (4, 2) Thread (0, 0) Thread (1, 0) Thread (2, 0) Thread (3, 0) Thread (4, 0) Courtesy: NDVIA 6 HK-UIUC

CUDA Device Memory Space Overview Each thread can: R/W per-thread registers R/W per-thread local memory R/W per-block shared memory R/W per-grid global memory Read only per-grid constant memory Read only per-grid texture memory (Device) Grid Constant Memory Texture Memory Global Memory Block (0, 0) Shared Memory Local Memory Thread (0, 0) Registers Local Memory Thread (1, 0) Registers Block (1, 0) Shared Memory Local Memory Thread (0, 0) Registers Local Memory Thread (1, 0) Registers Host The host can R/W global, constant, and texture memory 7 IMPORTANT NOTE: Global, constant, and texture memory spaces are persistent across kernels called by the same host application. HK-UIUC

Global, Constant, and Texture Memories (Long Latency Accesses by Host) Global memory Main means of communicating R/W Data between host and device Contents visible to all threads Texture and Constant Memories Constants initialized by host Contents visible to all threads (Device) Grid Constant Memory Texture Memory Global Memory Block (0, 0) Shared Memory Local Memory Thread (0, 0) Registers Local Memory Thread (1, 0) Registers Block (1, 0) Shared Memory Local Memory Thread (0, 0) Registers Local Memory Thread (1, 0) Registers Host Courtesy: NDVIA 8 HK-UIUC NOTE: We will not emphasize texture memory in this class.

9 End: Memory Layout on the GPU Begin: CUDA API

What is an API? Application Programming Interface (API) A set of functions, procedures or classes that an operating system, library, or service provides to support requests made by computer programs (from Wikipedia) Example: OpenGL, a graphics library, has its own API that allows one to draw a line, rotate it, resize it, etc. Cooked up analogy (for the mechanical engineer) Think of a car, you can say it has a certain Device Operating Interface (DOI): A series of pedals, gauges, handwheel, etc. This would be its DOI In this context, CUDA is the API that enables you to tap into the computational resources of the NVIDIA GPU This is what replaced the old GPGPU way of programming the hardware 10

Overview CUDA programming model – basic concepts and data types CUDA application programming interface - basic Simple example to illustrate basic concepts and functionality 11 HK-UIUC Performance features will be covered later

Talking about the API: The CUDA Software Stack Image at right indicates where the API fits in the picture 12 An API layer is indicated by a thick red line: Dealing with the CUDA Driver API is tedious We’ll only discuss the CUDA Runtime API, which handles all the dirty laundry for you (under the hood, it might deal with the CUDA Driver) Examples of CUDA Libraries: CUDA FFT and CUDA BLAS

CUDA Highlights: Easy and Lightweight The entire CUDA API is an extension to the ANSI C programming language Low learning curve The hardware is designed to enable lightweight runtime and driver High performance 13 HK-UIUC Here we go…

CUDA Device Memory Allocation (Device) Grid Constant Memory Texture Memory Global Memory Block (0, 0) Shared Memory Local Memory Thread (0, 0) Registers Local Memory Thread (1, 0) Registers Block (1, 0) Shared Memory Local Memory Thread (0, 0) Registers Local Memory Thread (1, 0) Registers Host 14 cudaMalloc() Global Memory Allocates object in the device Global Memory Requires two parameters Address of a pointer to the allocated object Size of allocated object cudaFree() Frees object from device Global Memory Pointer to freed object HK-UIUC

A Small Detour: A Matrix Data Type NOT part of CUDA It will be frequently used in many code examples 2 D matrix Single precision float elements Width * height elements Pitch meaningful when the matrix is actually a sub-matrix of another matrix Matrix entries attached to the pointer-to-float member called “elements” Matrix is stored row-wise typedef struct { int width; int height; int pitch; float* elements; } Matrix; 15 HK-UIUC

CUDA Device Memory Allocation (cont.) Code example: Allocate a 64 * 64 single precision float array Attach the allocated storage to Md.elements “d” is often used to indicate a device data structure BLOCK_SIZE = 64; Matrix Md; int size = BLOCK_SIZE * BLOCK_SIZE * sizeof(float); cudaMalloc((void**)&Md.elements, size); cudaFree(Md.elements); 16 HK-UIUC All the details are spelled out in the CUDA Programming Guide 1.1 (see the resources section of the class website) VERY USEFUL, PLEASE READ…

CUDA Host-Device Data Transfer cudaMemcpy() memory data transfer Requires four parameters Pointer to source Pointer to destination Number of bytes copied Type of transfer  Host to Host  Host to Device  Device to Host  Device to Device (Device) Grid Constant Memory Texture Memory Global Memory Block (0, 0) Shared Memory Local Memory Thread (0, 0) Registers Local Memory Thread (1, 0) Registers Block (1, 0) Shared Memory Local Memory Thread (0, 0) Registers Local Memory Thread (1, 0) Registers Host 17 HK-UIUC

CUDA Host-Device Data Transfer (cont.) Code example: Transfer a 64 * 64 single precision float array M is in host memory and Md is in device memory cudaMemcpyHostToDevice and cudaMemcpyDeviceToHost are symbolic constants cudaMemcpy(Md.elements, M.elements, size, cudaMemcpyHostToDevice); cudaMemcpy(M.elements, Md.elements, size, cudaMemcpyDeviceToHost); 18 HK-UIUC

CUDA Function Declarations Executed on the: Only callable from the: __device__ float DeviceFunc() device __global__ void KernelFunc() devicehost __host__ float HostFunc() host __global__ defines a kernel function Must return void __device__ and __host__ can be used together 19 HK-UIUC

CUDA Function Declarations (cont.) __device__ functions can’t have their address taken For functions executed on the device: No recursion No static variable declarations inside the function No variable number of arguments Something like printf would not work… 20 HK-UIUC

__global__ void KernelFunc(...); // declaration dim3 DimGrid(100, 50); // 5000 thread blocks dim3 DimBlock(4, 8, 8); // 256 threads per block size_t SharedMemBytes = 64; // 64 bytes of shared memory KernelFunc >>(...); Calling a Kernel Function, and the Concept of Execution Configuration A kernel function must be called with an execution configuration: 21 Any call to a kernel function is asynchronous from CUDA 1.0 on, explicit sync needed for blocking HK-UIUC