IORefs and garbage collection

[Ravi Nanavati]

I recently discovered that I'm running into the IORef / garbage
collection issue described in the ghc user guide (at the bottom of the
following page):

http://www.haskell.org/ghc/docs/6.4/html/users_guide/faster.html

Increasing the heap and allocation area size (with -H, -M and -A) helped
improved my runtimes considerably (by preventing all of my execution 
time from being sucked up by the garbage collector), but I have some 
questions:

1. What "goes wrong" with mutable data and generational garbage
collection to make this sort of workaround necessary? The issue 
surprised me because I thought there was plenty of satisfied experience 
with generational gc for languages with many more destructive updates 
(e.g. Java) than Haskell. Is there some optimization that the ghc 
runtime is not implementing?

2. If there is a known fix for this issue, what would it involve (and, 
if there are any guesses, how much work might it be)?

3. What is the best workaround for this issue if you *don't* know the 
maximum amount of memory available for your program? Would be it be best 
to fall back copying collection if you want your program to consume and 
release memory "as needed" or is there a cleverer trick?

Thanks,

  - Ravi