Renderscript c style pointer usage performance issue

Question

In render script, I am using bound pointers to iterate over a large image. The problem is in the array access performance.

...
for(int i=0; i < channels; i++) {
    sum += (input[i*input_size]) * mulValue;
}
...

For example, when the input_size is 12288 it takes 1.5 seconds to complete script, but when the input_size is 12280 it takes ~0.5 seconds.

What can cause such a mystery behavior?

Answer 1

Understanding the performance implications of what you write in RenderScript (or openCL) is complex.

Just writing it in RendersScript does not guarantee performance. Many times you encounter cache coherence issues when your memory access hop around.

Quite often it is better to structure the code as a series of kernels that process in a cache friendly manner.

Sorry if this is vague. You questing does not have enough details.