This post originated from an RSS feed registered with Ruby Buzz by Eigen Class. Original Post: The dangers of #undef_method, #instance_exec recalled for memleaking! Feed Title: Eigenclass Feed URL: http://feeds.feedburner.com/eigenclass Feed Description: Ruby stuff --- trying to stay away from triviality. Latest Ruby Buzz Posts Latest Ruby Buzz Posts by Eigen Class Latest Posts From Eigenclass

After reading this thread on ruby-talk, I reviewed my instance_exec implementation and found it totally unacceptable. I used #undef_method instead of #remove_method, what a fool I was! I'll first show why it can be a problem and then explain the causes; the problem is quite general, affecting a number of meta-hacks.

Take this script that just defines lots of methods to undefine them right away, measuring the VmSize as it goes*1

ITER = 10

def vm_size
  File.read("/proc/#{Process.pid}/status")[/^VmSize:\s+(\d+) kB/, 1].to_i
end

vm1 = vm_size
puts "I'm #{Process.pid}, using #{vm1} kB."
b = lambda{}
ITER.times do |i|
  (i*10000...(i+1)*10000).each do |i|
    name = "foo%6d" % i
    Object.module_eval{ define_method(name, &b); undef_method name }
  end
  GC.start
end
inc = vm_size - vm1
puts "undef_method"
puts "Increment: #{inc}, #{inc * 1024 / (ITER * 10000)} bytes per method."

Here's the output:

$ ruby undef_method.rb 
I'm 1343, using 3020 kB.
undef_method
Increment: 23092, 118 bytes per method.

The script is saying that each method definition is taking around 120 bytes, even when the method is removed right away.*2

This is not surprising: the memleak is caused by the symbols. But that's not all. I claimed that #undef_method was worse than #remove_method, and here's the proof: running the above script after substituting undef_method with remove_method yields this:

$ ruby remove_method.rb 
I'm 3170, using 3020 kB.
remove_method
Increment: 6932, 70 bytes per method.

The increment per method remains consistently at least 40+ bytes smaller than for #undef_method, so there's something besides symbol leaking.

How undef_method works

The difference lies in how remove_method and undef_method works.