CPU+GPU for rendering

How can we acheive this ? I want to use both CPU+GPU for rendering? I cannot find FP_No_HOST_CHECK in my environment variables.

You need to have a build that supports OpenCL for rendering. Try here http://graphicall.org/?keywords=opencl

Offical build supports OpenCL for rendering… that is what Cycles uses. As for the environment variable, you have to add it yourself.

But the acceleration when adding CPU to the GPU should not be noticeable, because the average GPU has 2,000 cores
and the average CPU has only 4/8 cores.

You can’t compare things that are not comparable. Depending on your CPU, it can be faster, equal, slower, a lot slower etc than your GPU. CPU core and GPU core are not the same thing and definitely not comparable by counting their numbers.

Currently you do not need to start blender that way. You can do it like this:

The thing is (at least in my tests), OpenCL on CPU is much slower than just CPU. I also think that Cycles is not optimized to use these two devices at the same time.
For animation, perhaps you should consider this (Using just CPU in one instance, and GPU in the other instance):

Anyway the result with CPU will be slower, Cycles uses a CPU thread per GPU.