The magic of clGetKernelWorkGroupInfo

Workgroup with unknown characteristics
Workgroup with unknown characteristics

It’s not easy to get the available private memory size – actually it’s impossible to get this information directly from the device/drivers, using the OpenCL API. This can only be explained after you dive deep into clGetKernelWorkGroupInfo – the function that tells you how well your kernel fits on the device. It is strange this function is not often discussed.

Memory sizes


Returns the amount of local memory, in bytes, being used by a kernel (per work-group). Use CL_DEVICE_LOCAL_MEM_SIZE to find out the maximum.


Returns the minimum amount of private memory, in bytes, used by each work-item in the kernel.

Work sizes


This answers the question “What is the maximum value for global_work_size argument that can be given to clEnqueueNDRangeKernel?”. The result is of type size_t[3].


The is the same for local_work_size. The kernel’s resource requirements (register usage etc.) are used, to determine what this work-group size should be.


If  __attribute__((reqd_work_group_size(X, Y, Z))) is used, then (X, Y, Z) is returned, else (0, 0, 0).


It returns a performance-hint: if the total number of work-items is a multiple of this number, then you’ll get good results. So no more remembering 32 or 64 for specific GPUs, but simply kick in a call to this function.

Combined with clDeviceInfo’s CL_DEVICE_MAX_WORK_ITEM_DIMENSIONS, you can fine-tune your workgroup-size in case you need the group-size to be as large as possible.

Read more?

You’ll find interesting usages when specifically looking for the flags on Github or Stackoverflow.

Short list of interesting Stackoverflow discussions: