Per-generation lists of weak pointers
marlowsd at gmail.com
Thu Mar 21 06:40:07 CET 2013
On 11/03/13 03:17, Akio Takano wrote:
> I'm working on implementing per-generation lists of weak pointers to
> speed up garbage collection in programs that allocate a lot of weak
> pointers. I have a patch  that validates and gives a 3x speed up on
> a benchmark. However I'd like to ask for some advise before finishing
> and submitting the patch.
>  https://github.com/takano-akio/ghc/commit/c7345c68eaa1e7f9572e693b5e352e386df7d680
> The problem is that since my patch splits the weak pointer list
> between generations, it no longer maintains the right order of weak
> pointers. This could cause finalizers added with
> addForeignPtrFinalizer to run in the wrong order.
> I can think of one way to fix it; to make sure that when a WEAK object
> gets promoted, it is always added to the front of the new list. So my
> questions are:
> - Would it be a correct fix?
> - If so, is it an acceptable fix? For example, is it too fragile a
> reasoning to rely on?
I don't like the way that we rely on the ordering of the weak pointer
list right now. I think this invariant arose accidentally when the
support for C finalizers was added. It was wrong for some time, see:
and as per my comments in that commit log, I think we should do it
differently. I don't know how hard that would be though.
Incidentally, I implemented per-generation weak pointers in the local-gc
branch, but didn't get around to porting it back over into the mainline
(I still have a ToDo for that). My version probably has the ordering
bug, but you could always look at the branch to see how my approach
compares to yours.
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