I'm currently optimizing an OpenCL kernel, and have been trying to find optimal values for workgroup sizes and vector widths.
Currently I'm using an ubuntu system with an Intel i7-3930k (6 cores @ 3.5 GHz, HT disabled) and an AMD HD6870. Both Intel and AMD OpenCL implementations are installed to allow for comparisons. ( APP SDK v2.7 Linux 64b & Catalyst 12.4, Intel OpenCL SDK 1.5 ).
Running on the CPU (on Intel Platform) I've found that:
- By choosing a wg size of 256 I can gain about 13.5% performance in comparison to wgsize=1.
- By specifying `__attribute__((vec_type_hint(float4)))` I can gain a 30% boost.
- By specifying `__attribute__((work_group_size_hint(WG_SIZE, 1, 1)))` I get another ~90% 
So, in total, theses options can result in close to a 3x performance increase. Unfortunately, when running this case on the the CPU using the AMD OpenCL platform, I've found that i can only get about 4% performance gain from specifying thw wgSize and the optional attributes are ignored.
Kernel declaration is:
kernel
__attribute__(( work_group_size_hint(WG_SIZE, 1, 1) ))
__attribute__(( vec_type_hint(VEC_SIZE) ))
void solveEikonalEq( global env_packed_t* env_packed_in,
global float* packedEnvData_in,
private float ds,
private float freq,
global ray_t* ray,
global rayMembers_t* rayMembers){
And compiler output is:
"/tmp/OCLVAvDqR.cl", line 2637: warning: unknown attribute "work_group_size_hint"
kernel __attribute__((work_group_size_hint(WG_SIZE, 1, 1)))
^
"/tmp/OCLVAvDqR.cl", line 2638: warning: unknown attribute "vec_type_hint"
__attribute__(( vec_type_hint(VEC_SIZE)))
^
Does AMD always ignore these hints? Or is there something i have to do to enable these attributes on the AMD platform?
From reading the opencl 1.1 spec i had the impression that supporting these kernel attributes was not optional (even if actually acting on them is)
