Per-generation lists of weak pointers

[Akio Takano]

Thank you very much for your help, I just updated the patches on the ticket.


On Sun, Apr 21, 2013 at 10:50 AM, Edward Z. Yang <ezyang at cs.stanford.edu>wrote:

> In your ticket, you mention this patch introduces a race condition.  One
> possible fix is to have addCFinalizerToWeak# check if the pointer is
> already
> dead, and just run the finalizer immediately if it is.  I think this
> preserves the semantics, but this needs to be checked closely.
>
> Edward
>
> Excerpts from Akio Takano's message of Fri Apr 19 02:58:50 -0700 2013:
> > I removed the invariant by adding a new primop, addCFinalizerToWeak#. I
> > opened a ticket for the issue.
> >
> > http://hackage.haskell.org/trac/ghc/ticket/7847
> >
> > - Akio
> >
> > On Thu, Mar 21, 2013 at 2:40 PM, Simon Marlow <marlowsd at gmail.com>
> wrote:
> >
> > > On 11/03/13 03:17, Akio Takano wrote:
> > >
> > >> Hi,
> > >>
> > >> 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 [1] 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.
> > >>
> > >> [1] https://github.com/takano-**akio/ghc/commit/**
> > >> c7345c68eaa1e7f9572e693b5e352e**386df7d680<
> 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:
> > >
> > > http://hackage.haskell.org/**trac/ghc/ticket/7160<
> http://hackage.haskell.org/trac/ghc/ticket/7160>
> > >
> > > 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.
> > >
> > > Cheers,
> > >         Simon
> > >
> > >
>
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