From: Joel VanderWerf <vjoel@...>
Date: 2007-05-23T10:57:13+09:00
Subject: Re: Confusing array garbage collection, how to get valid heap snapshot?

Greg Fodor wrote:
> We are working on tracking down a memory leak, and are hoping to use
> heap snapshotting to find the culprit. Using the neat bleak_house
> library:
> 
> http://blog.evanweaver.com/articles/2007/04/28/bleak_house
> 
> I have got snapshotting working on my local machine. Note this library
> does not go through ObjectSpaces, but uses a custom ruby build, so its
> much more accurate in showing the true state of the heap. However,
> there seems to be a bit of a problem, this simple script I wrote for
> testing purposes:
> 
> # Snapshot here
> a = Array.new(1000) { Object.new }
> a = nil
> 1.upto(20) { GC.start; ObjectSpace.garbage_collect; sleep 1; }
> # Snapshot here

This is probably not very helpful, because I don't know what bleak_house 
is but:

- calling GC repeatedly is unlikely to make a difference

- GC is conservative in treating stack data as potential pointers

- you can see the effect in the following example:

GC.start
n0 = ObjectSpace.each_object(Object){}

def foo
   a = Array.new(1000) { Object.new }
   a = nil

   a = Array.new(1000) { Object.new }
   a = nil

   a = Array.new(1000) { Object.new }
   a = nil
end

foo

GC.start
n1 = ObjectSpace.each_object(Object){}

p n1 - n0

__END__

This outputs 998 for me. If you only create one array inside of #foo, 
the output is the same, probably since the stack space was reused.

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407