gvl_release.rb

require 'fiddle'

class GVL
  handle  = Fiddle::Handle::DEFAULT
  address = handle['rb_thread_blocking_region']
  func = Fiddle::Function.new address, [Fiddle::TYPE_VOIDP,
                                        Fiddle::TYPE_VOIDP,
                                        Fiddle::TYPE_VOIDP,
                                        Fiddle::TYPE_VOIDP], Fiddle::TYPE_VOIDP

  define_method(:unlocked) do |&block|
    closure = Class.new(Fiddle::Closure) {
      define_method(:call) { |ptr|
        Fiddle.dlwrap block.call
      }
    }.new(Fiddle::TYPE_VOIDP, [Fiddle::TYPE_VOIDP])

    func.call(closure, nil, nil, nil).to_value
  end
end

def fib n
  if n < 3
    1
  else
    fib(n-1) + fib(n-2)
  end
end

gvl = GVL.new
p gvl.unlocked { fib(34) }

it's replaced with rb_thread_call_without_gvl according to @nobu