From: Sebastian Hungerecker <sepp2k@...>
Date: 2008-12-02T21:21:30+09:00
Subject: Re: Memory usage with blocks

John Ky wrote:
> When I replace the following code:
>
>                 head.play_back do |root|
>                    block.call(root)
>                 end
>
> With this:
>
>                 head.play_back(&block)
>
> And make other similar changes.  The program behaves exactly the same way,
> but now uses 200MB less.

Consider the following:

def x(&block)
  block
end

def a(&block)
  tmp = create_a_two_hundred_mega_byte_object()
  x {block.call} # Create a new block which closes over the current scope
end

def b(&block)
  tmp = create_a_two_hundred_mega_byte_object()
  x(&block) # Reuse the block that has been passed to this method.
            # Do not create a new block
end

If I call b {puts "hello"}, the following will happen:
The block {puts "hello"} is created. This block holds a reference to any local 
variable that has been defined in the scope in which I call method b.
Now method b is called with that block as an argument.
Then a 200mb object is created and stored in tmp.
Now x is called. x returns a Proc representing the block passed to b.
The method b ends, tmp goes out of scope and the 200mb object is garbage 
collected.

If I call a {puts "hello"}, this happens:
The block {puts "hello"} is created. This block holds a reference to any local 
variable that has been defined in the scope in which I call method a.
Now method a is called with that block as an argument.
Then a 200mb object is created and stored in tmp.
Now another block is created, which invokes the block passed to a.
This new block holds a reference to any local variable tmp defined in the 
method body of a. Specifically it holds a reference to tmp.
Now x is called. x returns a Proc representing this new block.
The method a ends, but the 200mb is still referenced by the Proc that's 
returned from a. So as long as reference to that Proc is stored, the 200mb 
object will not be garbage collected.

I don't know whether that's what happens in your code, but it's certainly 
worth looking into.

HTH,
Sebastian
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