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Cuda

• Useful references

  • nvida gencode
  • cuda architecture

• Hello World

  • Everything

// For all CUDA calls
void e(cudaError_t result) {
  if (result != cudaSuccess) {
    printf("%s\n", cudaGetErrorString(result));
  }
}

// Sanity checks after running a kernel
e(cudaGetLastError()); 
e(cudaDeviceSynchronize());

// Common functions
cudaMemcpy(dest, source, size, cudaMemcpyHostToDevice | cudaMemcpyDeviceToHost)
cudaMalloc(&<pointer>, size)
  
// Calling a kernel
kernel<<<blocks, threadsinblock>>>()
threadIdx.x
blockDim.x // threads in a block in that direction
gridDim.x // blocks in a grid in that direction

// Shared memory
 __shared__ int s[]; <- size defined by parameter to the kernel

• References
  • https://developer.nvidia.com/blog/how-query-device-properties-and-handle-errors-cuda-cc/
  • https://developer.nvidia.com/blog/easy-introduction-cuda-c-and-c/
  • GridDim/striding: https://developer.nvidia.com/blog/cuda-pro-tip-write-flexible-kernels-grid-stride-loops/
    • via https://stackoverflow.com/questions/16619274/cuda-griddim-and-blockdim
• [[Thread Block]]
  • https://developer.nvidia.com/blog/cuda-refresher-cuda-programming-model/
  • group of threads is called a thread block
  • CUDA blocks are grouped into a [[grid]]
  • Kernel is executed as a grid of blocks of threads
  • each block is executed by one [[Streaming Multiprocessor]] (SM)
  • One SM can run several CUDA blocks (depending on resources)
  • Cuda limits threads per block to 1024
  • =kernel_call<<<numBlocks, threadsPerBlock>>>(...)=
• [[Warp]]
  • Groups of threads with consecutive thread indexes are bundled into warps
  • One full warp is executed on a single cuda core
  • blocks are divided into multiple warps for execution on corers of an SM
  • [[SIMT]] -- single instruction, multiple threads
    • multiple threads issue common instructions to arbitrary data
    • each thread accesses its own registers, loads/stores from different addresses
  • https://developer.nvidia.com/blog/using-cuda-warp-level-primitives/
• [[Parallel Reduce]]
  • https://sodocumentation.net/cuda/topic/6566/parallel-reduction--e-g--how-to-sum-an-array-
  • https://developer.download.nvidia.com/assets/cuda/files/reduction.pdf
• Shared memory
  • https://developer.nvidia.com/blog/using-shared-memory-cuda-cc/
• Printf in Cuda
  • Pre-decides a buffer before kernel launch, and then will over-write it if exceeded
  • =cudaDeviceGetLimit(size_t *ptr, cudaLimitPrintfFifoSize)=, =cudaDeviceSetLimit(cudaLimitPrintfFifoSize, size_t )= to override
  • (Couldn't actually get this to behave how I'd hoped)

— Kunal