Does Global Work Size Need to be Multiple of Work Group Size in OpenCL?

Question

Hello: Does Global Work Size (Dimensions) Need to be Multiple of Work Group Size (Dimensions) in OpenCL?

If so, is there a standard way of handling matrices not a multiple of the work group dimensions? I can think of two possibilities:

Dynamically set the size of the work group dimensions to a factor of the global work dimensions. (this would incur the overhead of finding a factor and possibly set the work group to a non-optimal size.)

Increase the dimensions of the global work to be the nearest multiple of the work group dimensions, keeping all input and output buffers the same but checking bounds in the kernel to avoid segfaulting, i.e. do nothing on the work items out of bound of the desired output. (This seems like the better way.)

Would the second way work? Is there a better way? (Or is it not necessary because work group dimensions need not divide global work dimensions?)

Thanks!

Answer 1

Thx for the link Chad. But actually, if you read on:

If local_work_size is specified, the values specified in global_work_size[0], … global_work_size[work_dim - 1] must be evenly divisible by the corresponding values specified in local_work_size[0], … local_work_size[work_dim – 1].

So YES, the local work size must be a multiple of the global work size.

I also think the assigning the global work size to the nearest multiple and being careful about bounds should work, I'll post a comment when I get around to trying it.