I'm not an expert on how the GI algorithms are parallelized. First, it's worth noting there are a lot of them, so the strategy probably varies. But here's a gue

I'm not an expert on how the GI algorithms are parallelized. First, it's worth noting there are a lot of them, so the strategy probably varies. But here's a guess for a popular one known as photon mapping. With that technique, you bounce around virtual photons, and they contribute to the lighting of each point they hit (to simplify a bit). AFAIK, each photon's path is only a function of the light source, the scene geometry, and the scene materials. I.e. it's not a function of what other photons are doing. So I think you can parallelize individual photons bouncing around. As a final step, you have to fold all the light contributions together, which I believe could in turn be parallelized per polygon. But I'm just guessing about all this.