Re: BDW-GC branch updated

[Andy Wingo <wingo@pobox.com>]

Hi Andreas,

On Tue 18 Aug 2009 14:19, Andreas Rottmann <a.rottmann@gmx.at> writes:

> 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.

I'm sure they do.

So, as you're probably aware, Guile's GC right now is something of a
hybrid. It conservatively scans the stack, but precisely marks the heap.
BDW scans the heap with less precision. Of course there are pointerless
blocks and custom mark functions in BDW as well.

I was concerned about this issue myself. Of course a moving GC is a
better option in the long run. But it seems an impossible change to
make, given Guile's long and public history as a C library.

BDW-GC is simply a better conservative GC than Guile's current
conservative GC.

But to take a specific example: I don't want Guile to be the project
that broke Emacs. Emacs has a precise GC, though not a moving GC iirc,
and if conservative collection broke long-running emacs sessions, that
would be Bad. But we shouldn't have any problem with Emacs. There will
be almost nothing on the C stack for any length of time. We control all
of the heap as well. We have pointerless mark functions in place where
necessary. Only tagged values make it onto the VM stack. So it is highly
unlikely to ever hit those corner cases PLT seemed to hit all the time.

Anyway, need to head out. Your concerns are important ones. My opinion
is that BDW-GC sounds like the right choice for the next 10 years of
Guile.

Andy
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