Re: BDW-GC branch updated

[ludo@gnu.org (Ludovic Courtès)]

Hi,

Andreas Rottmann <a.rottmann@gmx.at> writes:

> ludo@gnu.org (Ludovic Courtès) writes:

[...]

>> So now is a good time to test it and report back!  It requires libgc 7.1
>> or later, which isn't packaged in Debian, although it was released in
>> May 2008.
>>
> It's in experimental since recently; I assume its maintainer will upload
> to unstable soonish.

Good.

> Will going from a precise GC to BDW-GC not have drawbacks? IIRC, the PLT
> people went in the opposite direction. A quick google turned up this:
>
> http://www.cs.brown.edu/pipermail/plt-scheme/2006-June/013840.html
>
> I wonder if the reasons for switching to a precise GC listed in there
> will also apply to Guile.

Thanks for the link!

They write:

  There is one known problem, though, related to linked lists [Boehm,
  POPL'02].  Unfortunately, we seem to hit this problem often in
  practice, due to the way that threads and continuations are
  implemented, and there doesn't seem to be a reliable way around it.

The paper is "Bounding Space Usage of Conservative Garbage Collectors",
available from http://www.hpl.hp.com/personal/Hans_Boehm/gc/ .  It
depicts scenarios where "false references" lead to unbounded data
retention.  My interpretation of these scenarios and the "Summary"
section is that these cases are hopefully quite rare.

Now, I don't have enough experience of long-running BDW-GC applications
to know whether it's a problem in practice.  The PLT folks surely had
more experience (but with a different implementation IIUC).  There are
also other schemes that use BDW-GC, such as Bigloo.

However, it doesn't worry me as much as the current GC bugs (e.g., [0, 1]).

Also, there are definite benefits to using a conservative GC for
libguile, given how tightly it can be integrated with C (e.g., [2]).

What do you think?

Thanks,
Ludo'.

[0] http://permalink.gmane.org/gmane.lisp.guile.devel/6832
[1] http://lists.gnu.org/archive/html/guile-devel/2008-08/msg00120.html
[2] http://lists.gnu.org/archive/html/guile-devel/2008-11/msg00009.html