Not all work-items being used opencl - c++

Question

so I'm able to compile and execute my kernel, the problem is that only two work-items are being used. I'm basically trying to fill up a float array[8] with {0,1,2,3,4,5,6,7}. So this is a very simple hello world application. Bellow is my kernel.
// Highly simplified to demonstrate
__kernel void rnd_float32_matrix (
__global float * res
) {
uint idx = get_global_id(0);
res[idx] = idx;
}
I then create and execute the kernel with the following code...
// Some more code
cl::Program program(context, sources, &err);
program.build(devices, NULL, NULL, NULL);
cl::Kernel kernel(program, "rnd_float32_matrix", &err);
kernel.setArg(0, src_d);
cl::CommandQueue queue(context, devices[0], 0, &err);
cl::Event event;
err = queue.enqueueNDRangeKernel(
kernel,
cl::NullRange,
cl::NDRange(8),
// I've tried cl::NDRange(8) as well
cl::NDRange(1),
NULL,
&event
);
event.wait();
err = queue.enqueueReadBuffer(
// This is:
// cl::Buffer src_d(
// context,
// CL_MEM_READ_ONLY | CL_MEM_COPY_HOST_PTR,
// mem_size,
// src_h,
// &err);
src_d,
CL_TRUE,
0,
8,
// This is float * src_h = new float[8];
src_h);
for(int i = 0; i < 8; i ++) {
std::cout << src_h[i] << std::endl;
}
I may not show it in the code, but I also do select a gpu device and using context.getInfo(..) it shows I'm using my NVidia GTX 770M card which shows 1024, 1024, 64 work-items available in dimensions 0, 1 and 2. When this array prints I keep getting... 0, 1, 0, 0, 0, 0, 0, 0. I've also tried setting res[idx] = 5, and I get... 5, 5, 0, 0, 0, 0, 0, 0. So it seems that only two give work-items are actually being used. What am I doing wrong?

Answer 1

Your command to read the data back from the device is only reading 8 bytes, which is two floats:
err = queue.enqueueReadBuffer(
src_d,
CL_TRUE,
0,
8, // <- This is the number of bytes, not the number of elements!
// This is float * src_h = new float[8];
src_h);
To read 8 floats, you would need to do this:
err = queue.enqueueReadBuffer(
src_d,
CL_TRUE,
0,
8 * sizeof(cl_float),
// This is float * src_h = new float[8];
src_h);