Thank you for your comments! On Wed, Apr 24, 2013 at 8:23 PM, Edward Z. Yang wrote:

Great, good to see.

Some questions:

- You seem to be stubbing out cfinalizers with stg_WEAK_IS_DEAD_closure when a weak pointer gets finalized; your locking is predicated on operations on this field being cas'es. Another design is to lock the closure itself, via the info table header (using lockClosure). I think both strategies should be sound, but I suspect lockClosure is bound to be safer, in case you end up mutating other fields.

You are right, I updated the patches to use lockClosure. I was not aware of the existence of lockClosure when I wrote the previous patches, which is why I used cas().

- Typo "Kill a weak poitner"

Now the comment is gone.

- There was a comment about LDV which was removed. Why is that comment irrelevant in the new world order?

In that version of the patches finalizeWeak# didn't modify the infoptr, so the comment didn't apply. It is back in the latest version of the patches, though.

Edward

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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

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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:

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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

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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

Excerpts from Akio Takano's message of Wed Apr 24 02:12:42 -0700 2013: 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

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