#7662: Improve GC of mutable objects ---------------------------------+------------------------------------------ Reporter: ezyang | Owner: Type: feature request | Status: new Priority: normal | Milestone: _|_ Component: Runtime System | Version: 7.7 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: None/Unknown Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ Changes (by simonmar): * difficulty: => Unknown * milestone: => _|_ Comment: One problem with card marking is that you need to be able to traverse the heap linearly to scan the objects in a card. GHC's heap doesn't support this, because we have gaps between objects left behind when a thunk is overwritten by an indirection. It's possible to fill in the gaps (we do this when `DEBUG` is enabled), but it's a fair bit of overhead on every update. This is why we only have card marking for arrays. We should have a small array type that uses the same write barrier as a `MutVar#`, i.e. it isn't on the mutable list until it is modified, and there's no card marking. This would help in the `unordered-containers` package I believe. I've often wondered whether we can fully unpack an `IORef`, that is have a mutable pointer field in an ordinary constructor. It would be nice if we could do it, but I fear it is hard to achieve - you can't re-box a mutable field like you can with an immutable field. Perhaps we could represent a `MutVar#` as a pointer plus an offset instead. (I'm leaving the milestone for this at _|_, which is not to say that none of this will ever happen, just that we don't have concrete plans right now). -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7662#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler