[plt-scheme] minimizing garbage collection time

More GC questions...  Here are typical running times for two of my
benchmark functions:

cpu time: 19300 real time: 19386 gc time: 18270
cpu time: 58860 real time: 60695 gc time: 53500

If I'm reading this right, this means that if I could turn off the
garbage collector, these would only take 1 and 5 seconds,
respectively.  Is that right?  Is it abnormal for garbage collection
to take 90-95% of the time?

What triggers the garbage collector?  Is it whenever the allocation
pool is empty?  Or is garbage collected as soon as it becomes garbage?

I have a lot of weak hash tables, but maybe I have some non-weak
tables that should also be weak-- would holding onto too many objects
that should otherwise be garbage cause the collector to do a lot of
extra traversals that it shouldn't need to do?

Any other tips for reducing GC time (or even just measuring it better)
would be appreciated.

--dougo at place.org