Cuda gridDim and blockDim

• blockDim.x,y,z gives the number of threads in a block, in the
  particular direction
• gridDim.x,y,z gives the number of blocks in a grid, in the
  particular direction
• blockDim.x * gridDim.x gives the number of threads in a grid (in the x direction, in this case)

block and grid variables can be 1, 2, or 3 dimensional. It’s common practice when handling 1-D data to only create 1-D blocks and grids.

In the CUDA documentation, these variables are defined here

In particular, when the total threads in the x-dimension (gridDim.x*blockDim.x) is less than the size of the array I wish to process, then it’s common practice to create a loop and have the grid of threads move through the entire array. In this case, after processing one loop iteration, each thread must then move to the next unprocessed location, which is given by tid+=blockDim.x*gridDim.x; In effect, the entire grid of threads is jumping through the 1-D array of data, a grid-width at a time. This topic, sometimes called a “grid-striding loop”, is further discussed in this blog article.

You might want to consider taking an introductory CUDA webinars For example, the first 4 units. It would be 4 hours well spent, if you want to understand these concepts better.