#15642: Improve the worst case performance of weak pointers -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.8.1 Component: Runtime System | Version: 8.6.1-beta1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by osa1): Somehow marking keys and being able map keys to weaks would be nice. As you say the mapping should be able to map a key to multiple weaks because an object may be key to multiple weaks. Out of curiosity, did you measure how long weak collection takes? I'd guess because we scan all weaks in collected gens repeatedly (until we stop evacuating things) the worst case performance (when we have a ton of weaks) would be pretty bad, but I haven't benchmarked this case myself.
This is the weakest link: if we don't have a spare bit there, or don't want to use one, then I think this whole idea is sunk.
I think in theory it is possible to use a bit in info pointer. We already do this to create forwarding pointers in GC, we could use a different bit to indicate that the objects is a weak key. This however requires changes in mutator code as mutators will have to untag info ptr before entering (in addition to untagging the pointer to the object itself in some cases). That's probably not great as we end up generating more code and probably making things slightly slower (one more instruction to enter -- not sure if measurable difference though) to be able to collect weaks faster, and a lot of programs don't have a lot of weaks. I wonder how other languages with weaks (Python, Java, Lua ...) do this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15642#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler