[Haskell] garbage collection of Concurrent Haskell threads?

Simon Marlow simonmarhaskell at gmail.com
Wed Jan 2 09:14:34 EST 2008

[ moving to glasgow-haskell-users at haskell.org ]

Conal Elliott wrote:
> Thanks, Simon.  If I understand the mechanism you're describing, it 
> discards readers of an empty MVar when there are no other references to 
> the MVar *because* the MVar can never get written.  And if there are 
> other readers but no writers, then I'm guessing GC wouldn't know that, 
> and none of the readers get discarded.  Is that so?

I'm not sure I understand precisely what you mean here.  Do you mean when 
there are other running threads that might read the MVar in the future?  In 
that case, yes, blocked readers would not be detected and sent an exception.

> I think Baker & Hewitt's trick was analogous to discarding writers of an 
> already full MVar when there are readers (readMVar) but no takers 
> (takeMVar).  (Though not quite, since readMVar is implemented via 
> takeMVar & putMVar.)

It shouldn't be the case that you have a thread doing putMVar on a full 
MVar simultaneously with another thread doing readMVar, because the 
readMVar could be preempted between the take and put and then block - this 
would almost certainly be an undesirable race condition.

> I guess that effectively means IVars instead of MVars.
> In either direction (blocked reader or blocked writer), the interface of 
> MVars (or IVars) would seem to prevent an accurate analysis, since GC 
> wouldn't know whether a Var reference was for reading or writing.
> Right?  A simple solution might be to hide the Var itself and instead 
> expose reader and writer halves.  If there's an analysis problem at all, 
> does that solution make sense?

Absolutely.  It ought to be possible to implement this using weak pointers 
in GHC, because you want relationships like "if a writer is alive, then the 
readers are alive", and this is the kind of thing you can express with weak 


> Cheers,  - Conal
> On Dec 24, 2007 1:02 AM, Simon Peyton-Jones <simonpj at microsoft.com 
> <mailto:simonpj at microsoft.com>> wrote:
>     GHC already garbage-collects threads that are blocked on an MVar
>     that is otherwise inaccessible (and hence cannot be updated).  More
>     precisely, GHC sends the thread an asynchronous exception
>     (ThreadBlocked or something), so that it has a chance to clean up.
>     So perhaps the GC you want is already implemented?
>     Simon
>     *From:* haskell-bounces at haskell.org
>     <mailto:haskell-bounces at haskell.org>
>     [mailto:haskell-bounces at haskell.org
>     <mailto:haskell-bounces at haskell.org>] *On Behalf Of *Conal Elliott
>     *Sent:* 24 December 2007 00:15
>     *To:* haskell at haskell.org <mailto:haskell at haskell.org>
>     *Subject:* [Haskell] garbage collection of Concurrent Haskell threads?
>     The classic paper "The Incremental Garbage Collection of Processes"
>     (http://citeseer.ist.psu.edu/baker77incremental.html) describes
>     "futures" and how particularly garbage collecting them when their
>     pending result is no longer referenced.  I've been playing with an
>     implementation of futures in Concurrent Haskell (
>     http://haskell.org/haskellwiki/Reactive), using MVars, and I'm
>     stumped about how to GC non-winning threads in a race between
>     futures ("parallel or").  I'm having winner kill loser, which seems
>     to work fine, though is potentially dangerous w.r.t locked
>     resources.  Still, the elegance of a GC-based solution appeals to
>     me.  Has anyone explored process GC ideas for Concurrent Haskell (or
>     STM)?
>     Futures are implemented using Concurrent Haskell's MVars.  I first
>     tried using STM and TVars, simply using orElse to implement mappend
>     for futures.  However, I didn't see how to avoid nesting
>     "atomically", which yielded a run-time error.  If anyone has ideas
>     about using STM & TVars for futures, I'd love to hear.
>     Thanks,  - Conal
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