kernelQueryFunctions(3) Manual Page

uint get_kernel_work_group_size(void (^block)(void))
uint get_kernel_work_group_size(void (^block)(local {localArgType} *, …​))

This provides a mechanism to query the maximum work-group size that can be used to execute a block on a specific device given by device.

block specifies the block to be enqueued.

uint {get_kernel_preferred_work_group_size_multiple}( void (^block)(void))
uint {get_kernel_preferred_work_group_size_multiple}( void (^block)(local {localArgType} *, …​))

Returns the preferred multiple of work-group size for launch. This is a performance hint. Specifying a work-group size that is not a multiple of the value returned by this query as the value of the local work size argument to enqueue_kernel will not fail to enqueue the block for execution unless the work-group size specified is larger than the device maximum.