GC: Generation 3 questions

Hello,

I have a delivered application, and its generation 3 (4th generation)
area is growing fast.

LispWorks manual says:

"Generation 3 normally contains only objects that existed at startup
time, that is those were saved in the image. Normally it is not scanned
at all, except when an image is saved."

I guess the application is not normal somehow. My questions are:

* When the generation 3 area can grow?
* Is there any way to GC generation 3? I found
'collect-highest-generation' function, but don't know how/when actual GC
for generation 3 happens(I called GC, but it didn't collect anything in
generation 3).
* How to prevent the growing?

Thanks

- Jong-won Choi