Performance of atomics

Atomics in OpenCL are very useful, but if they are not used carefully, severe performance penalties can appear. Let's create simple OpenCL kernel which does sum of ones utilizing atomics:

kernel void AtomicSum(global int* sum){
    atomic_add(sum,1);
}

Let's try to test this kernel running 1024x1024x128 threads:

int sum=0;
cl::Buffer bufferSum = cl::Buffer(context, CL_MEM_READ_WRITE, 1 * sizeof(float));
queue.enqueueWriteBuffer(bufferSum, CL_TRUE, 0, 1 * sizeof(int), &sum);
cl::Kernel kernel=cl::Kernel(program, "AtomicSum");
kernel.setArg(0,bufferSum);
queue.enqueueNDRangeKernel(kernel, cl::NullRange, cl::NDRange(1024*1024*128), cl::NullRange);
queue.finish();

queue.enqueueReadBuffer(bufferSum,CL_TRUE,0,1 * sizeof(int),&sum);
std::cout << "Sum: " << sum << "\n";