[Haskell-cafe] IORef memory leak

Thomas Schilling nominolo at googlemail.com
Fri Oct 15 19:16:01 EDT 2010

Correct, here's a video of Simon explaining the thunk blackholing
issue and its solution in GHC 7:


On 15 October 2010 21:31, Gregory Collins <greg at gregorycollins.net> wrote:
> Evan Laforge <qdunkan at gmail.com> writes:
>> The only workaround I could find is to immediately read the value back
>> out and 'seq' on it, but it's ugly.
> Yep! C'est la vie unfortunately.
> The way atomicModifyIORef works is that the new value isn't actually
> evaluated at all; GHC just swaps the old value with a thunk which will
> do the modification when the value is demanded.
> It's done like that so that the atomic modification can be done with a
> compare-and-swap CPU instruction; a fully-fledged lock would have to be
> taken otherwise, because your function could do an unbounded amount of
> work. While that's happening, other mutator threads could be writing
> into your memory cell, having read the same old value you did, and then
> *splat*, the souffle is ruined.
> Once you're taking a lock, you've got yourself an MVar. This is why
> IORefs are generally (always?) faster than MVars under contention; the
> lighter-weight lock mechanism means mutator threads don't block, if the
> CAS fails atomicModifyIORef just tries again in a busy loop. (I think!)
> Of course, the mutator threads themselves then tend to bump into each
> other or do redundant work when it's time to evaluate the thunks (GHC
> tries to avoid this using "thunk blackholing"). Contention issues here
> have gotten radically better in recent versions of GHC I think.
> Forgive me if I've gotten anything wrong here, I think Simon Marlow
> might be the only person who *really* understands how all this stuff
> works. :)
>> So two questions:
>> writeIORef doesn't have this problem.  If I am just writing a simple
>> value, is writeIORef atomic?  In other words, can I replace
>> 'atomicModifyIORef r (const (x, ())' with 'writeIORef r x'?
>> Any reason to not do solution 1 above?
> Well if you're not inspecting or using the old value then it's safe to
> just blow it away, yes.
> Cheers,
> G
> --
> Gregory Collins <greg at gregorycollins.net>
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Push the envelope. Watch it bend.

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