From: ts <decoux@...>
Date: 2007-04-24T18:39:23+09:00
Subject: Re: Tracking down a garbage collection problem

>>>>> "W" == Wincent Colaiuta <win@wincent.com> writes:

W> One thing which is puzzling me in the meantime is why GC doesn't seem
W> to work in the following example:

 Well, you must understand that the GC is conservative, this mean that it
 can mark object which are not used any more by ruby, and more it work at
 assembler level (sort of) when you look at ruby level.

 Here your example on *my* system

moulon% cat b.rb
#!/usr/bin/ruby
def finalizer(thing)
  puts "setting up finalizer for #{thing}"
  lambda { puts "finalizer for #{thing}" }
end

def meth
  foo = "hello, world!"
  ObjectSpace.define_finalizer(foo, finalizer(foo.object_id))
end

meth
puts "about to start GC"
GC.start
puts "did start GC"

meth
puts "about to start GC"
GC.start
puts "did start GC"

moulon% 

moulon% ./b.rb
setting up finalizer for -605724018
about to start GC
did start GC
setting up finalizer for -605722868
about to start GC
did start GC
finalizer for -605724018
finalizer for -605722868
moulon% 

 When the GC run, it find the 2 objects on the C stack and mark it, now if
 it write it like this

moulon% diff -u b.rb~ b.rb
--- b.rb~	2007-04-24 11:34:04.000000000 +0200
+++ b.rb	2007-04-24 11:34:21.000000000 +0200
@@ -9,12 +9,12 @@
   ObjectSpace.define_finalizer(foo, finalizer(foo.object_id))
 end
 
-meth
+1.times { meth }
 puts "about to start GC"
 GC.start
 puts "did start GC"
 
-meth
+1.times { meth }
 puts "about to start GC"
 GC.start
 puts "did start GC"
moulon% 

moulon% ./b.rb
setting up finalizer for -605810148
about to start GC
did start GC
setting up finalizer for -605808898
about to start GC
finalizer for -605810148
did start GC
finalizer for -605808898
moulon% 

 and worst

moulon% diff -u b.rb~ b.rb
--- b.rb~	2007-04-24 11:34:04.000000000 +0200
+++ b.rb	2007-04-24 11:35:15.000000000 +0200
@@ -9,12 +9,12 @@
   ObjectSpace.define_finalizer(foo, finalizer(foo.object_id))
 end
 
-meth
+1.times { 1.times { meth } }
 puts "about to start GC"
 GC.start
 puts "did start GC"
 
-meth
+1.times { 1.times { meth } }
 puts "about to start GC"
 GC.start
 puts "did start GC"
moulon% 

moulon% ./b.rb
setting up finalizer for -605462098
about to start GC
finalizer for -605462098
did start GC
setting up finalizer for -605461048
about to start GC
finalizer for -605461048
did start GC
moulon% 

 I've just made in sort to call #meth with enough stack to make in sort
 that the GC don't find the variable in the stack.when it run.

 But remember this is on *my* system and it can give different results on
 *your* system and *this is NOT important*


Guy Decoux