Web servers handling lots of small requests are actually pretty easy to garbage collect to: you just delete all the data at the end of the request.
Either you have a specialised GC that works like this, or probably a good general generational GC can pick up on this pattern on its own.
Or you do as Erlang's BEAM VM: each thread has it's own memory area which is GC'ed individually. This means upon request termination, you just terminate the thread and the memory is reclaimed with no need for a GC.
In the abstract, this is very similar to spawning a unix process for every request, never free-ing any memory, and letting the memory allocation die with the process.