I suspect trial samples uses the first X number of GI samples as guidance for future GI samples. Meaning its limited by the amount of GI samples you have, so if you set GI samples to 512 and trial samples to 512, it essentially does nothing. I’m guessing the trial samples may need to be kept in GPU memory also so perhaps that is causing the crash for you.
My testing seems to bear out this conclusion perfectly:
Following that logic, I gather there is a probably “sweet spot” for trial sample count, where by continuing to increase the value just gives you a worse result because there aren’t enough regular GI samples to take advantage of all the trial samples.
Not sure where the tipping point is (intuitively I want to say 50% as that seems to make sense), but I think it is a good idea to use the defaults as a basis for determining what to set. By default the Trial Samples are 25% of the GI samples (128 trial samples / 512 GI samples) and I find this works pretty well. So if you were doing 1024 GI samples, you’d use 256 trial samples. In my case for the comparison in my previous post, I used 8192 GI samples and 2048 trial samples…
First bounce also seems to have some bugs, I sometimes I get blocky artifacts and broken results between tiles. Not sure why.