As some of you may have heard, we're considering disabling 
ObjectSpace.each_object by default in JRuby. Primarily, this is for 
performance; to support each_object, we have to bend over backwards, 
maintaining lists of weak references to all objects in the system and 
periodically cleaning out those lists. Here's some example performance, 
from a fractal benchmark in the JRuby source:

With ObjectSpace: Ruby Elapsed 45.967000
Without ObjectSpace: Ruby Elapsed 4.280000

What's most frustrating about this is that almost *no* libraries or apps 
use each_object, and it's a terrible performance hit for us.

The one really visible use of each_object is in test/unit, where the 
default console-based runner does each_object(Class) to find all 
subclasses of TestCase. Because this is a heavily-used library (to say 
the least), I've made modifications to JRuby to always support 
each_object(Class) by maintaining a bidirectional graph of parent and 
child classes. So that much wouldn't go away (but I'd prefer an 
implementation that uses Class#inherited, since it would be cleaner, 
faster, and deterministic).

So...I'm writing this to see what the general Ruby world thinks of us 
having ObjectSpace disabled by default, enableable via a command line 
option (or perhaps through a library? -robjectspace?).

I think more and more of you may want to give JRuby another look over 
the next few months, so I think we need to involve you in such decisions.

- Charlie