[Haskell] garbage collection of Concurrent Haskell threads?

Marnix Klooster Marnix.Klooster at infor.com
Thu Jan 3 10:02:25 EST 2008

Hello Simon,
For the record: I assume that by "Boehm's article on finalisers" you
  author    = {Hans-Juergen Boehm},
  title     = {Destructors, finalizers, and synchronization},
  booktitle = {POPL},
  year      = {2003},
  pages     = {262-272},
  ee        = {http://doi.acm.org/10.1145/640128.604153},
  bibsource = {DBLP, http://dblp.uni-trier.de}
(which is also at http://citeseer.ist.psu.edu/boehm03destructors.html)?

Marnix Klooster | Software Engineer, ERP LN Enterprise Server | Infor |
(+31 or 0)342-428511 | marnix.klooster at infor.com | Infor Global
Solutions (Barneveld) BV | P.O. box 143 | 3770 AC Barneveld | The



	From: haskell-bounces at haskell.org
[mailto:haskell-bounces at haskell.org] On Behalf Of Simon Peyton-Jones
	Sent: Thursday, 3 January, 2008 14:13
	To: Conal Elliott
	Cc: haskell at haskell.org
	Subject: RE: [Haskell] garbage collection of Concurrent Haskell

	The GC discards an MVar when there are only blocked readers on
it.   But there can be any number of blocked readers.  So long as the
read remains in the future, however, the GC doesn't recover it, because
it has no way to know that the thread will only read it and not write


	Your idea of splitting the MVar into two halves, a read end and
a write end, might well improve this situation somewhat.


	But, like finalisers, it's dangerous to rely on reachability
arguments for anything that's really important.  Boehm's article on
finalisers explains why.  My guess is that the same remarks would apply
to MVars.




	From: conal.elliott at gmail.com [mailto:conal.elliott at gmail.com]
On Behalf Of Conal Elliott
	Sent: 24 December 2007 18:49
	To: Simon Peyton-Jones
	Cc: haskell at haskell.org
	Subject: Re: [Haskell] garbage collection of Concurrent Haskell


	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 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.)  I guess that effectively means IVars instead of
	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? 
	Cheers,  - Conal

	On Dec 24, 2007 1:02 AM, Simon Peyton-Jones
<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?


	From: 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
	Subject: [Haskell] garbage collection of Concurrent Haskell


	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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