CUDA device에서 제공하는 메모리의 종류는 다음과 같다.

5.3.2  Device Memory Accesses .................................................................... 70 
    5.3.2.1  Global Memory ............................................................................ 70 
    5.3.2.2  Local Memory .............................................................................. 72 
    5.3.2.3  Shared Memory ........................................................................... 72 
    5.3.2.4  Constant Memory ........................................................................ 73 
     5.3.2.5  Texture and Surface Memory ........................................................ 73  

[출처 :  CUDA C Programming guide.pdf] 

Local memory 와 Global memory는 그래픽 카드의 비디오 메모리(통상 512MB / 1기가 이런식으로 말하는)에 존재하고
Shared memory는 GPU 내의 Multi-Processor에 통합되어있다.

Devicequery를 비교하면서 보자면
8800GT 512MB 짜리에서
Global memory와 Local memory는 512MB 까지 가능하며
Shared memory는 블럭당 16KB 까지 가능하다.

 Device 0: "GeForce 8800 GT"
  CUDA Driver Version:                           3.20
  CUDA Runtime Version:                          3.10
  CUDA Capability Major revision number:         1
  CUDA Capability Minor revision number:         1
  Total amount of global memory:                 536543232 bytes
  Number of multiprocessors:                     14
  Number of cores:                               112
  Total amount of constant memory:               65536 bytes
  Total amount of shared memory per block:       16384 bytes
  Total number of registers available per block: 8192
  Warp size:                                     32
  Maximum number of threads per block:           512
  Maximum sizes of each dimension of a block:    512 x 512 x 64
  Maximum sizes of each dimension of a grid:     65535 x 65535 x 1   

2011/01/02 - [Programming/openCL / CUDA] - deviceQuery on 8600GT 512MB + CUDA 하드웨어 구조
      
 

예제로 들어있는 행렬곱 예제에서
shared memory를 사용하고 사용하지 않는 차이점은 아래의 그림처럼
Global memory에 직접 한 바이트씩 읽어서 계산하는지

아니면 global memory의 블럭을
shared memory로 일정 영역만(블럭 사이즈 만큼) 복사해서 계산을 하는지의 차이점이 있다.

다른 책에 의하면 global memory는 700~900 cuda clock에 읽어오고
shared memory는 거의 1 cuda clock에 읽어 온다고 하니
되도록이면 shared memory에 복사해서 더욱 빠르게 연산하는게 유리하다고 한다.

 

 

// Matrices are stored in row-major order: 
// M(row, col) = *(M.elements + row * M.width + col) 
typedef struct { 
    int width; 
    int height; 
    float* elements; 
} Matrix; 
 
// Thread block size 
#define BLOCK_SIZE 16 
 
// Forward declaration of the matrix multiplication kernel 
__global__ void MatMulKernel(const Matrix, const Matrix, Matrix); 
 


























 
// Matrix multiplication - Host code 
// Matrix dimensions are assumed to be multiples of BLOCK_SIZE 
void MatMul(const Matrix A, const Matrix B, Matrix C) 
    // Load A and B to device memory 
    Matrix d_A; 
    d_A.width = A.width; d_A.height = A.height; 
    size_t size = A.width * A.height * sizeof(float); 
    cudaMalloc(&d_A.elements, size); 
    cudaMemcpy(d_A.elements, A.elements, size, 
               cudaMemcpyHostToDevice); 
    Matrix d_B; 
    d_B.width = B.width; d_B.height = B.height; 
    size = B.width * B.height * sizeof(float); 
    cudaMalloc(&d_B.elements, size); 
    cudaMemcpy(d_B.elements, B.elements, size, 
               cudaMemcpyHostToDevice); 
 
    // Allocate C in device memory 
    Matrix d_C; 
    d_C.width = C.width; d_C.height = C.height; 
    size = C.width * C.height * sizeof(float); 
    cudaMalloc(&d_C.elements, size); 
 
    // Invoke kernel 
    dim3 dimBlock(BLOCK_SIZE, BLOCK_SIZE); 
    dim3 dimGrid(B.width / dimBlock.x, A.height / dimBlock.y); 
    MatMulKernel<<<dimGrid, dimBlock>>>(d_A, d_B, d_C); 
 
    // Read C from device memory 
    cudaMemcpy(C.elements, Cd.elements, size, 
               cudaMemcpyDeviceToHost); 
 
    // Free device memory 
    cudaFree(d_A.elements); 
    cudaFree(d_B.elements); 
    cudaFree(d_C.elements); 

// Matrix multiplication kernel called by MatMul() 
__global__ void MatMulKernel(Matrix A, Matrix B, Matrix C) 
    // Each thread computes one element of C 
    // by accumulating results into Cvalue 
    float Cvalue = 0; 
    int row = blockIdx.y * blockDim.y + threadIdx.y; 
    int col = blockIdx.x * blockDim.x + threadIdx.x; 
    for (int e = 0; e < A.width; ++e) 
        Cvalue += A.elements[row * A.width + e] 
                * B.elements[e * B.width + col]; 
    C.elements[row * C.width + col] = Cvalue; 













































 

// Matrices are stored in row-major order: 
// M(row, col) = *(M.elements + row * M.stride + col) 
typedef struct { 
    int width; 
    int height; 
    int stride;  
    float* elements; 
} Matrix; 
 
// Get a matrix element 
__device__ float GetElement(const Matrix A, int row, int col) 
    return A.elements[row * A.stride + col]; 
 
// Set a matrix element 
__device__ void SetElement(Matrix A, int row, int col, 
                           float value) 
    A.elements[row * A.stride + col] = value; 
 
// Get the BLOCK_SIZExBLOCK_SIZE sub-matrix Asub of A that is 
// located col sub-matrices to the right and row sub-matrices down 
// from the upper-left corner of A 
__device__ Matrix GetSubMatrix(Matrix A, int row, int col)  
    Matrix Asub; 
    Asub.width    = BLOCK_SIZE; 
    Asub.height   = BLOCK_SIZE; 
    Asub.stride   = A.stride; 
    Asub.elements = &A.elements[A.stride * BLOCK_SIZE * row 
                                         + BLOCK_SIZE * col]; 
    return Asub; 
 
// Thread block size 
#define BLOCK_SIZE 16 
 
// Forward declaration of the matrix multiplication kernel 
__global__ void MatMulKernel(const Matrix, const Matrix, Matrix); 
 
// Matrix multiplication - Host code 
// Matrix dimensions are assumed to be multiples of BLOCK_SIZE 
void MatMul(const Matrix A, const Matrix B, Matrix C) 
    // Load A and B to device memory 
    Matrix d_A; 
    d_A.width = d_A.stride = A.width; d_A.height = A.height; 
    size_t size = A.width * A.height * sizeof(float); 
    cudaMalloc(&d_A.elements, size); 
    cudaMemcpy(d_A.elements, A.elements, size, 
               cudaMemcpyHostToDevice); 
    Matrix d_B; 
    d_B.width = d_B.stride = B.width; d_B.height = B.height; 
    size = B.width * B.height * sizeof(float); 
    cudaMalloc(&d_B.elements, size); 
    cudaMemcpy(d_B.elements, B.elements, size, 
               cudaMemcpyHostToDevice); 
 
    // Allocate C in device memory 
    Matrix d_C; 
    d_C.width = d_C.stride = C.width; d_C.height = C.height; 
    size = C.width * C.height * sizeof(float); 
    cudaMalloc(&d_C.elements, size); 
 
    // Invoke kernel 
    dim3 dimBlock(BLOCK_SIZE, BLOCK_SIZE); 
    dim3 dimGrid(B.width / dimBlock.x, A.height / dimBlock.y); 
    MatMulKernel<<<dimGrid, dimBlock>>>(d_A, d_B, d_C); 
 
    // Read C from device memory 
    cudaMemcpy(C.elements, d_C.elements, size, 
               cudaMemcpyDeviceToHost); 
 
    // Free device memory 
    cudaFree(d_A.elements); 
    cudaFree(d_B.elements); 
    cudaFree(d_C.elements); 
 
// Matrix multiplication kernel called by MatMul() 
__global__ void MatMulKernel(Matrix A, Matrix B, Matrix C) 
    // Block row and column 
    int blockRow = blockIdx.y; 
    int blockCol = blockIdx.x; 
 
    // Each thread block computes one sub-matrix Csub of C 
    Matrix Csub = GetSubMatrix(C, blockRow, blockCol); 
     // Each thread computes one element of Csub 
    // by accumulating results into Cvalue 
    float Cvalue = 0; 
 
    // Thread row and column within Csub 
    int row = threadIdx.y; 
    int col = threadIdx.x; 
 
    // Loop over all the sub-matrices of A and B that are 
    // required to compute Csub 
    // Multiply each pair of sub-matrices together 
    // and accumulate the results 
    for (int m = 0; m < (A.width / BLOCK_SIZE); ++m) { 
 
        // Get sub-matrix Asub of A 
        Matrix Asub = GetSubMatrix(A, blockRow, m); 
 
        // Get sub-matrix Bsub of B 
        Matrix Bsub = GetSubMatrix(B, m, blockCol); 
 
        // Shared memory used to store Asub and Bsub respectively 
        __shared__ float As[BLOCK_SIZE][BLOCK_SIZE]; 
        __shared__ float Bs[BLOCK_SIZE][BLOCK_SIZE]; 
 
        // Load Asub and Bsub from device memory to shared memory 
        // Each thread loads one element of each sub-matrix 
        As[row][col] = GetElement(Asub, row, col); 
        Bs[row][col] = GetElement(Bsub, row, col); 
 
        // Synchronize to make sure the sub-matrices are loaded 
        // before starting the computation 
        __syncthreads(); 
 
        // Multiply Asub and Bsub together 
        for (int e = 0; e < BLOCK_SIZE; ++e) 
            Cvalue += As[row][e] * Bs[e][col]; 
 
        // Synchronize to make sure that the preceding 
        // computation is done before loading two new 
        // sub-matrices of A and B in the next iteration 
        __syncthreads(); 
    } 
 
    // Write Csub to device memory 
    // Each thread writes one element 
    SetElement(Csub, row, col, Cvalue); 
}


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