Can some people who understand weak references &c take a look at this PR: https://github.com/apache/hadoop/pull/3085
It's aim is to remove a lock in Configuration construction, so could speed construction up. It does get fairly complicated though, which is why it needs many eyeballs. We've hit scale issues with Configuration in the recent past (different hive threads creating FS instances). I don't think this was where the problems surfaced, but given how broadly Configuration is used: (a) better scale is good and (b) we don't want to break it -steve
