Even with the GVL, this counter ends well below 20000. What does that prove about the GVL's guarantees?

class Counter attr_accessor :n def initialize = @n = 0 def bump v = @n Thread.pass @n = v + 1 end end c = Counter.new 20.times.map { Thread.new { 1000.times { c.bump } } }.each(&:join) c.n # => far less than 20000