Maybe this will help someone, as well...we've been trying to collect all of the different tuning and implementation information on the Ruby GC in one place:
That's actually a huge help. What I've also been looking for (and haven't been able to find) is a good guide on How to Tune your GC settings. Aman Gupta has gotten the closest so far* (I think), but I'd really love a step-by-step of "OK, open your console, run `GC.start`, take note of this property because it means X and indicates that Y would be a good setting for Z, etc. etc"
It would be interesting or useful to know more about it, but what I think is that ideally a right impementation of a GC should not require any tweeks from the outside... and I am pretty sure you are not going to get any signifcant performance boost by using different GC values than default.
* http://tmm1.net/ruby21-rgengc/