blockDim

blockDim is a built-in variable in CUDA programming that provides information about the dimensions of a thread block. When a kernel is launched in CUDA, it is organized into a grid of thread blocks, and each block is further divided into threads. The blockDim variable is a structure that holds the size of a thread block in each of the three possible dimensions: x, y, and z. Specifically, blockDim.x refers to the number of threads in the x-dimension of the block, blockDim.y for the y-dimension, and blockDim.z for the z-dimension.

This variable is crucial for parallel programming in CUDA because it allows threads within a block to understand their local context and coordinate their execution. For example, threads can use blockDim.x to calculate unique thread indices within their block, which is essential for accessing data in shared memory or for implementing various parallel algorithms. The values of blockDim are determined at kernel launch time and are constant for all threads within a given block. Understanding and utilizing blockDim correctly is fundamental for efficient GPU computation and for correctly mapping computational problems onto the CUDA hardware.