First CUDA Program

#include "stdio.h" int main() { printf("Hello, world\n"); return 0; } #include __global__ void kernel (void) { } int main (void) { kernel >> (); printf("Hello World!\n"); return 0; } First C program First CUDA program Compilation nvcc -o first first.cu./first Compilation gcc -o first first.c./first

Kernels CUDA C extends C by allowing the programmer to define C functions, called kernels, – when called, are executed N times in parallel by N different CUDA threads, as opposed to only once like regular C functions. A kernel is defined using the _ _global_ _ declaration specifier. The number of CUDA threads that execute that kernel for a given kernel call is specified using a new >> execution configuration syntax Amrita School of Biotechnology

Example Program 1 “__global__” says the function is to be compiled to run on a “device” (GPU), not “host” (CPU) Angle brackets “ >>” for passing params/args to runtime #include __global__ void kernel (void) { } int main (void) { kernel >> (); printf("Hello World!\n"); return 0; } A function executed on the GPU (device) is usually called a “kernel” Amrita School of Biotechnology

Example Program 2 We can pass parameters to a kernel as we would with any C function _ _global_ _ void add(int a, int b, int *c) { *c = a+b; } int main (void) { int c, *dev_c; cudaMalloc ((void **) &dev_c, sizeof (int)); add >> (2,7, dev_c); cudaMemcpy(&c, dev_c, sizeof(int), cudaMemcpyDeviceToHost); printf(“2 + 7 = %d\n“, c); cudaFree(dev_c); return 0; } We need to allocate memory to do anything useful on a device BlocksPerGrid, threadsPerBlock Amrita School of Biotechnology

CUDA Device Memory Allocation cudaMalloc() : cudaError_t cudaMalloc ( void ** devPtr, size_t size ) Global Memory – Allocates object in the device Global Memory – Allocates size bytes of linear memory on the device and returns in *devPtr a pointer to the allocated memory. – Requires two parameters Address of a pointer to the allocated object Size of allocated object cudaFree() cudaError_t cudaFree ( void * devPtr ) – Frees object from device Global Memory Pointer to freed object Grid Global Memory Block (0, 0)‏ Shared Memory Thread (0, 0)‏ Registers Thread (1, 0)‏ Registers Block (1, 0)‏ Shared Memory Thread (0, 0)‏ Registers Thread (1, 0)‏ Registers Host Amrita School of Biotechnology

CUDA Device Memory Allocation (cont.)‏ Code example: –Allocate a 64 * 64 single precision float array –Attach the allocated storage to Md –“d” is often used to indicate a device data structure TILE_WIDTH = 64; Float* Md int size = TILE_WIDTH * TILE_WIDTH * sizeof(float); cudaMalloc((void**)&Md, size); cudaFree(Md); Amrita School of Biotechnology

Thread Per-thread Local Memory Block Per-block Shared Memory Memory model Kernel 0... Per-device Global Memory... Kernel 1 Sequential Kernels Device 0 memory Device 1 memory Host memory cudaMemcpy () There are also two additional read-only memory spaces accessible by all threads: the constant and texture memory spaces. The global, constant, and texture memory spaces are persistent across kernel launches by the same application. The CUDA programming model assumes that both the host and the device maintain their own separate memory spaces in DRAM, referred to as host memory and device memory, respectively.

Amrita School of Biotechnology Each thread can: – Read/write per-thread registers – Read/write per-thread local memory – Read/write per-block shared memory – Read/write per-grid global memory – Read/only per-grid constant memory Grid Global Memory Block (0, 0) Shared Memory Thread (0, 0) Registers Thread (1, 0) Registers Block (1, 0) Shared Memory Thread (0, 0) Registers Thread (1, 0) Registers Host Constant Memory

CUDA Host-Device Data Transfer cudaMemcpy() : –memory data transfer cudaError_t cudaMemcpy ( void * dst, const void * src, size_t count, enum cudaMemcpyKind kind ) –Requires four parameters Pointer to destination Pointer to source Number of bytes copied Type of transfer –Host to Host, cudaMemcpyHostToHost –Host to Device: cudaMemcpyHostToDevice –Device to Host: cudaMemcpyDeviceToHost –Device to Device : cudaMemcpyDeviceToDevice Grid Global Memory Block (0, 0)‏ Shared Memory Thread (0, 0)‏ Registers Thread (1, 0)‏ Registers Block (1, 0)‏ Shared Memory Thread (0, 0)‏ Registers Thread (1, 0)‏ Registers Host Amrita School of Biotechnology

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, M, size, cudaMemcpyHostToDevice); cudaMemcpy(M, Md, size, cudaMemcpyDeviceToHost); Amrita School of Biotechnology

Summing Vectors A simple example to illustrate threads and how we use them to code with CUDA C. Amrita School of Biotechnology

#define N 10 void add( int *a, int *b, int *c ) { int tid = 0; // this is CPU zero, so we start at zero while (tid < N) { c[tid] = a[tid] + b[tid]; tid += 1; // we have one CPU, so we increment by one } int main( void ) { int a[N], b[N], c[N]; for (int i=0; i<N; i++) { // fill the arrays 'a' and 'b' on the CPU a[i] = -i; b[i] = i * i; } add( a, b, c ); // display the results for (int i=0; i<N; i++) { printf( "%d + %d = %d\n", a[i], b[i], c[i] ); } return 0; } Traditional C code in CPU: Amrita School of Biotechnology

GPU Vector Sums We can accomplish the same addition very similarly on a GPU by writing add() as a device function. #define N 10 int main( void ) { int a[N], b[N], c[N]; int *dev_a, *dev_b, *dev_c; // allocate the memory on the GPU cudaMalloc( (void**)&dev_a, N * sizeof(int) ) ; cudaMalloc( (void**)&dev_b, N * sizeof(int) ) ; cudaMalloc( (void**)&dev_c, N * sizeof(int) ) ; // fill the arrays 'a' and 'b' on the CPU for (int i=0; i<N; i++) { a[i] = -i; b[i] = i * i; } // copy the arrays 'a' and 'b' to the GPU cudaMemcpy( dev_a, a, N * sizeof(int),cudaMemcpyHostToDevice ) ; cudaMemcpy( dev_b, b, N * sizeof(int), cudaMemcpyHostToDevice ) ; add >>( dev_a, dev_b, dev_c ); Amrita School of Biotechnology

// copy the array 'c' back from the GPU to the CPU cudaMemcpy( c, dev_c, N * sizeof(int), cudaMemcpyDeviceToHost ) ; // display the results for (int i=0; i<N; i++) { printf( "%d + %d = %d\n", a[i], b[i], c[i] ); } // free the memory allocated on the GPU cudaFree( dev_a ); cudaFree( dev_b ); cudaFree( dev_c ); return 0; } // Kernel definition __global__ void add( int *a, int *b, int *c ) { int tid = blockIdx.x; // handle the data at this index if (tid < N) c[tid] = a[tid] + b[tid]; } Amrita School of Biotechnology kernel callable from host __global__ void KernelFunc(...); function callable on device __device__ void DeviceFunc(...); variable in device memory __device__ int GlobalVar; in per-block shared memory __shared__ int SharedVar;

In – add >>( dev_a, dev_b, dev_c ); N is the number of blocks that we want to run in parallel. – If we call add >>(..), the function will have four copies running in parallel, where each copy is named a block. Thread block = a (data) parallel task – all blocks in kernel have the same entry point – but may execute any code they want Amrita School of Biotechnology

This is what the actual code being executed in each of the four parallel blocks looks like after the runtime substitutes the appropriate block index for blockIdx.x: Runtime system is already launching a different kernel where each block will have one of these indices, the work is done in parallel. Amrita School of Biotechnology