frame rate drops by GI (realtime & baked)

I get very bad frame rates when using GI - what's the trick to fix this?

 

You shouldn't. How bad is 'very bad'? V Sync is usually the cause of my framerate issues. Have you used the profiler?

 

In the project I'm working on my framerate is fine according to all stats I have access to (Fraps, the stats window and the profiler all claim I have hundreds of frames per second), but it's still choppy with realtime GI if I rotate my light. If I stop rotating the light the choppiness also stop. Something about constantly updating the bounced light seems to be causing choppiness. I made a note about it here:
http://forum.unity3d.com/threads/apartment-architectural-visualization.314694/#post-2055151

I haven't found a solution yet.

As for baked GI I have no idea. I would expect that to be virtually free. Isn't it just a lightmap?

 

I am seeing the same thing even with pretty simple scenes. It manifests as extreme variance in GPU performance, where every few frames there is a big consistent spike, with no spikes at all in the CPU profiler. This suggests to me that there is some very severe GPU-CPU sync point, bad enough that it is completely unusable for continuously moving light sources.

Perhaps Unity is not double buffering the GI data on the GPU? It would also be nice if we had scripting control over when the GI gets updated, like with reflection probes, then we could avoid doing other expensive infrequent rendering tasks in the same frame, or spread the GPU upload over several frames. Those sorts of optimizations are super critical when doing something this expensive and complicated.