[gclist] Experience with conservative GC sought

Michael Sperber [Mr. Preprocessor] sperber@Informatik.Uni-Tuebingen.De
16 Jan 1998 09:41:51 +0100

I'm looking for accounts, if part of research or just anecdotal, of
experience with using conservative GC in long-running applications
with large heaps and big changes in the size of the live data.

Both the Emacs and XEmacs development team have long-term plans to
replace their Lisp engines.  One of the big design issues is whether
to commit to conservative GC.  The present C substrates of these two
editors presently use precise GC annotations which are a constant
source of annoying bugs.  However, the present Emacsen have very small
leakage which typically allows one invocation to stay up for weeks or
even months.  Losing this property would be very undesirable.

Hans Boehm has provided me with his positive experience with the Cedar =

system, and a few other applications.

My own experience is mainly with Scheme implementations.

Rice's PLT suite of Scheme implementation uses the Boehm collector.  All
of them leak heavily, even when the size of the live data demonstrably =

stays almost constant.  It is usually necessary to restart interactive =

sessions every hour or so because they have grown beyond 40 Megabytes.

scm uses its own conservative collector which only scans the stack
conservatively but does precise tracing for objects inside the heap.
I have used scm to run programs with (comparatively) high rates of
data allocation.  The experience has been similar as with PLT: The
heap grows far beyond the size of the live data and doesn't shrink
back.  Moreover, as the size of the live data within a huge heap was
small, fragmentation caused substantial thrashing; the OS would
typically try to keep the whole heap in memory.  Note that GNU Guile
uses scm's collector.

The only published account I have of this kind of thing is:

  title =3D        "Pitfalls of Conservative Garbage Collection",
  author =3D       "E. P. Wentworth",
  journal =3D      "Software Practice and Experience",
  publisher =3D    "Wiley",
  volume =3D       "20",
  number =3D       "7",
  pages =3D        "719--727",
  year =3D         "1990",
  abstract =3D     "Researchers have recently proposed conservative
                 garbage collection as a technique which allows smooth
                 integration of automatic memory management into
                 existing high-level languages. Conservative techniques
                 carry the cost of some leakage --- they fail to reclaim
                 all the available garbage. This paper reports on a
                 conservative collector and measurements of its leakage.
                 The results indicate that the loss depends critically
                 on the application =3D {"}a Lisp interpreter shows very
                 small leakage (typically less than 5 per cent of the
                 total heap space for classroom examples), whereas the
                 garbage collector collapses completely when used with a
                 KRC interpreter. Uncovering the reasons reveals that it
                 is possible to write Lisp programs that also fail.
                 Previous work has used very sparsely populated address
                 spaces where the probability for leakage was minimized.
                 This work considers small, densely populated address
                 spaces, and probably represents a worst-case scenario.
                 Although the conservative technique appears to be a
                 cost-effective and clean way of integrating automatic
                 memory management into an off-the-shelf host language,
                 we are led to conclude that it is not reliable in all =

}  =

-- =

Cheers =3D8-} Mike
Friede, V=F6lkerverst=E4ndigung und =FCberhaupt blabla