1 ITCS 4/5010 CUDA Programming, UNC-Charlotte, B. Wilkinson, Jan 28, 2013, 3-DBlocks.ppt Addressing 2-D grids with 3-D blocks Class Discussion Notes

2 Given two-dimensional addressing, row, column Use the general 2-D to 1-D flattening equation: index = col + row * N col where N col is the total number of columns in a row to get a unique ID. Equation can be used multiple times in 2D/3D grids/blocks to get unique thread ID. General Approach

3 Thread blockIdx.x blockIdx.y Block Grid threadID.x threadID.y x y We have already considered 2-D grids and 2-D blocks

4 Applicable when mapping 2-D data array onto grid. Determine number of threads to the chosen thread, row and column: col = blockIdx.x*blockDim.x+threadIdx.x row = blockIdx.y*blockDim.y+threadIdx.y Then use: ThreadID = col + row * N where N = no of columns of threads in grid. N = blockDim.x * gridDim.x Substituting we would get: ThreadID = (blockIdx.x*blockDim.x+threadIdx.x) + (blockIdx.y*blockDim.y+threadIdx.y)* (blockDim.x * gridDim.x) = blockIdx.x*blockDim.x+threadIdx.x+ blockIdx.y*blockDim.y* blockDim.x * gridDim.x + threadIdx.y*blockDim.x *gridDim.x Global thread ID – one approach

5 Using the general 2-D to 1-D flattening equation: index = column + row * N column Block ID within grid: blockID = blockIdx.x + blockIdx.y * gridDim.x Thread ID within block: BlockthreadID = threadIdx.x + threadIdx.y * blockDim.x Then substitute BlockthreadID and blockID into flattening equation again to get threadID Global thread ID - Another approach

6 2-D Grids and 3-D blocks threadID.z Thread blockIdx.y Block Grid threadID.y threadID.x blockIdx.x

7 We have global thread ID not considering z direction. Call it threadID xy Using the general 2-D to 1-D flattening equation: index = col + row * N col threadID = threadID.z + threadID xy * blockDim.z 3D Global thread ID One approach

8 2-D Address, (x, y) and block sizes D x and D y Unique global thread ID = x + y D x 3-D Address, (x, y, z) and block sizes D x, D y, and D z. Unique global thread ID = x + y D x + zD x D y 3D Global thread ID Another approach

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