When does GC run?

[John Burton]

Is there any documentation or information about the specifics of 
the garbage collector?

The information I have found indicates that it runs to free 
memory when the system runs out of memory to allocate. But will 
this try to use all the system memory or some other amount before 
trying to garbage collect? If I have a _lot_ of garbage, will it 
try to collect that before allocating any more system memory? Is 
this configurable in any way?

Any information or just pointer to information that I've somehow 
failed to find would be appreciated.