了解 CL_DEVICE_MAX_WORK_GROUP_SIZE 限制 OpenCL？

Question

I have little bit difficulty understanding max work group limit reported by OpenCL and how it affects the program.

So my program is reporting following thing,

   CL_DEVICE_MAX_WORK_ITEM_SIZES  : 1024, 1024, 1024
   CL_DEVICE_MAX_WORK_GROUP_SIZE  : 256
   CL_DEVICE_MAX_WORK_ITEM_DIMENSIONS : 3

Now I am writing program to add vectors with 1 million entries. So the calculation for globalSize and localSize for NDRange is as follows

   int localSize = 64;
   // Number of total work items - localSize must be devisor
   globalSize = ceil(n/(float)localSize)*localSize;

 .......

    // Execute the kernel over the entire range of the data set 
    err = clEnqueueNDRangeKernel(queue, kernel, 1, NULL, &globalSize, &localSize,
                                                              0, NULL, NULL);

Here as per my understanding OpenCL indirectly calculates the number of work groups it will launch. For above example

globalSize = 15625 * 64 -> 1,000,000 -> So this is total number of threads that will be launched 
localSize = 64 -> So each work group will have 64 work items 

Hence from above we get

Total Work Groups Launched = globalSize/ localSize -> 15625 Work Groups 

Here my confusion starts, If you see value reported by OpenCL CL_DEVICE_MAX_WORK_GROUP_SIZE: 256 So, I was thinking this means max my device can launch 256 work groups in one dimension,

but above calculations showed that I am launching 15625 work groups.

So how is this thing working?

I hope some one can clarify my confusion. I am sure I am understanding something wrong.

Thanks in advance.

Answer 1

According to the specification of clEnqueueNDRangeKernel : https://www.khronos.org/registry/OpenCL/sdk/2.2/docs/man/html/clEnqueueNDRangeKernel.html , CL_DEVICE_MAX_WORK_ITEM_SIZES and CL_DEVICE_MAX_WORK_GROUP_SIZE indicate the limits of local size ( CL_KERNEL_WORK_GROUP_SIZE is CL_DEVICE_MAX_WORK_GROUP_SIZE in OpenCL 1.2).

const int dimension = n;
const int localSizeDim[n] = { ... }; // Each element must be less than or equal to 'CL_DEVICE_MAX_WORK_ITEM_SIZES[i]'
const int localSize = localSizeDim[0] * localSizeDim[1] * ... * localSizeDim[n-1]; // The size must be less than or equal to 'CL_DEVICE_MAX_WORK_GROUP_SIZ'

I couldn't find the device limit of global work items, but maximum value representable by size t is the limit of global work items in the description of the error CL_INVALID_GLOBAL_WORK_SIZE .