Blogmark

I had no idea Ruby hashes had this much of a memory footprint (relative to struct).

It’s super impressive to see the small steady improvements the Ruby core team / contributors are able to squeeze out by doing hyper-specific, low-level optimizations.

Reading this post reminded me of some of the Aaron Patterson talks I’ve seen.

I don't have much practical use for this kind of under-the-hood perf optimization analysis in my day-to-day. However, I do find it interesting to see the bits of Ruby code one can use to pull out these kinds of memory footprint numbers -- primarily ObjectSpace.memsize_of.

Here is some of my own basic fiddling:

> require 'objspace'
=> true
> ObjectSpace.memsize_of(1)
=> 0
> ObjectSpace.memsize_of("Hello, World!")
=> 40
> ObjectSpace.memsize_of([1,2,3])
=> 40
> ObjectSpace.memsize_of({[1,2,3] => :abc})
=> 160

And here is a snippet of measuring code from the post:

require 'objspace'
puts "Ruby: #{RUBY_VERSION}"

11.times do |size|
  struct_class = size.zero? ? Object : Struct.new(*size.times.map { |i| :"m_#{i}" })
  struct = ObjectSpace.memsize_of(struct_class.new)
  hash = ObjectSpace.memsize_of(Hash[size.times.map { |i| [i, i] }])
  diff = (hash.to_f / struct).round(1)
  puts "size: #{size} \tstruct: #{struct} \thash: #{hash} \tdiff: #{diff}x"
end