Slow sleep in combination with thread

I have a problem that the moment I open a thread, the sleep call becomes
totally unreliable.

require ‘thread’

3.times {
d = Thread.new{
while true

    end
}
d.run

}

puts sleep 0.01
puts “sleep ended”

The line sleep 0.01 sleeps a little over 10 seconds

ruby version:
ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-linux]

Describe what you think d.run does.