Re: Per-generation lists of weak pointers

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 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

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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 wrote:

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On 11/03/13 03:17, Akio Takano wrote:

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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/c7345c68eaa1e7f9572e693b5e352e386d...

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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

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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

Excerpts from Akio Takano's message of Fri Apr 19 02:58:50 -0700 2013: list mainline (I

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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