Opencl and HD5850

Question

I dont have an HD5850 but how can I know maximum workgroup size of it for opencl ? What is the preferred floating point vector width for HD5850? I suspected it was 5 but did not work on a friends computer who has 5850. Tried width 4 but did not work fast enough now I suspect work group size is not optimal. Doing NBody for 25k 50k and 100k particles consists of float8 variables for x,y,z, vx,vy,vz.

Thanks.

Answer 1

Use clGetDeviceInfo to poll for CL_DEVICE_MAX_WORK_GROUP_SIZE. I think the 5850 will have this at 256, but that may not be optimal for your kernel.

Use the same technique to poll for CL_DEVICE_PREFERRED_VECTOR_WIDTH_FLOAT, which I think is 4 on your card.

clGetDeviceInfo

Answer 2

If you need the OpenCL specifics at development time but don't have access to the hardware, try http://clbenchmark.com. For example, the HD 5850 page is here: http://clbenchmark.com/device-environment.jsp?config=11975982. It shows CL_DEVICE_PREFERRED_VECTOR_WIDTH_FLOAT=4.