[gclist] Wiring up the Boehm GC to the Ruby interpreter

Matthew Bloch matthew@bytemark.co.uk
Fri, 12 Jul 2002 11:36:41 +0100

Hi there;

I tried wiring up the Boehm GC (downloaded a couple of days ago) to the 
latest Ruby (from CVS) in an effort to smooth out the interpter's lengthy 
delays in an interactive application written in the language: the only 
changes I made to the Ruby source were:

* changed malloc/realloc/free to call GC_ equivalents like this in ruby.h:

   #ifdef HAVE_BOEHM_GC
   # include <gc/gc.h>
   # undef malloc
   # undef free
   # undef realloc
   # define malloc(x) GC_malloc(x)
   # define free(x) GC_free(x)
   # define realloc(x,y) GC_realloc(x,y)

* disabled Ruby signal handlers for SIGBUS/SIGSEGV;
* disabled all the code in rb_gc() and replaced with GC_collect_a_little();
* GC.enable / .disable / .start methods changed to affect the Boehm GC.

Only problem was-- it leaks!  Or at least the memory use from my application 
went up and up while the standard Ruby collector kept it bounded at around 
10%.  I could tell it was doing *something* because the memory shot up much 
more quickly if I turned off collection.  Are there any 'roots' in the Ruby 
interpreter that an external GC should need to be told about?  There were no 
shared library dependencies other than the usual libc etc.  Can this *only* 
be because there's memory Boehm doesn't know about, or are there other 
pitfalls to watch out for?

Also, I was a little disappointed not to see any difference 'smoothness' of 
garbage collection between the two GCs-- the Boehm GC still paused the 
application as often despite much fiddling with the global variables that can 
control the frequency & severity of incremental collcetion; is a realistic 
strategy to try to call GC_collect_a_little() every animation frame, with the 
time limit set to, say 10ms?  It didn't seem to stick to this unfortunately 
(though I realise this isn't a guarantee by the collector-- will 10ms always 
be too quick for it to do any useful work?).

Seeing as I'm fairly new to both the Ruby interpreter internals and the Boehm 
GC, I'd appreciate tips on using either.


Matthew Bloch                 Bytemark Computer Consulting
                                  tel. +44 (0) 8707 455026