From: Brian Candler <b.candler@...>
Date: 2008-09-17T22:23:14+09:00
Subject: Re: Threads preventing garbage collection?

Robert Klemme wrote:
>> For some reason, in the following code the instances of object Foo are
>> prevented from being garbage collected.
> 
> No, they are not collected.  This is something different!

OK, please consider my question rephrased as "why are these objects not 
collected?"

Let me increase the number of objects:

  def make_thread
    Thread.new { sleep 10000 }
  end

  class Foo
    attr_accessor :bar
  end

  10000.times { |i|
    f = Foo.new
    f.bar = make_thread
  }

  GC.start
  sleep 1
  GC.start

  count = 0
  ObjectSpace.each_object(Foo) { |o| count += 1 }
  puts "#{count} objects"

On my machine, this program uses about 150MB of RAM, and all 10,000 
objects are remaining at the end of the run.

Have I done something here which is "wrong"? Are the known pitfalls of 
the garbage collector documented anywhere, or ways to write programs so 
as to avoid them? Or can it simply not be relied upon, ever?

Regards,

Brian.
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