CGU vs CPU question

Some FcpX connoisseur stated to me that with FcpX the CPU is used for renders in the Timeline and the GPU for exports...

The next day an other "guru " was positive about ... the other way around !

So which is it ?

Either CPU or GPU can be used for rendering. As a gross generalization, the GPU is used for effects and the CPU for everything else. However some effects are CPU-oriented or even configurable between CPU and GPU. Neat Video is a good example of that. It can be configured to use all CPU, all GPU or a mix of both.

It is easy to see if a compute-intensive effect is GPU-oriented: apply it to a long clip and render that clip in the timeline with CTRL+R, then look at Activity Monitor's multi-core display or similar display in the 3rd party iStat Menus. If all CPU cores are high, then it is not GPU-bound, nor I/O-bound but CPU bound. iStat menus has a GPU activity and memory display which can give more info.

Exporting consists of two phases, rendering and encoding. Rendering (if effects are used) tends to involve the GPU, whereas encoding is typically a more CPU-oriented task. However if there are unrendered GPU-oriented effects in the timeline you export, the GPU will be involved during the render phase.

When it comes to FCPX and Motion 5, their system requirements suggest that they are based on OpenCL which allows you to pick between CPU and GPU devices for doing your calculations. Usually work is being divided between the two (or more, depending on your configuration), although many tasks can be run in parallel - in such case the GPU will perform much better.

That being said, FCPX and Motion 5 do a lot of parallel work so having a solid graphics card with at least 2GB of video memory is always a plus (most modern plugins will benefit from it as well). Intel is still far behind, it "steals" memory from your RAM and its current support is limited, to say the least. Investing in a decent dedicated/discrete graphics would be your best bet