Why is EM.epoll slowing down my connections in other threads?

bm.rb

#!/usr/bin/env ruby

# copied from https://gist.github.com/939696, and
# edited to add redis (which is where I was experiencing the issue)

require 'rubygems'
require 'net/http'
require 'hiredis'
require "redis/connection/hiredis"
require 'redis'
require 'eventmachine'
require 'benchmark'

redis = Redis.new
bm = lambda do
  http_get = 5.times.map do
    "(%.4fs)" % [Benchmark.realtime{ Net::HTTP.get URI.parse('http://www.google.com') }]
  end
  puts "HTTP.get:   #{http_get.join(" ")}"
  redis_ping = 5.times.map do
    "(%.4fs)" % [Benchmark.realtime{ redis.ping }]
  end
  puts "redis.ping: #{redis_ping.join(" ")}"
end

puts "### Before EM.run ###"
bm.call

puts "### EM.run #{ARGV[0] == "epoll" ? "with" : "without"} epoll ###"
Thread.new do
  EM.epoll if ARGV[0] == "epoll"
  EM.run do
    puts "### EM started ###"
  end
end
bm.call
EM.stop

puts "### EM stopped ###"
bm.call

with-epoll-output-1.9.3p0.txt

### Before EM.run ###
HTTP.get:   (0.1281s) (0.0965s) (0.0639s) (0.0956s) (0.0813s)
redis.ping: (0.0008s) (0.0002s) (0.0001s) (0.0001s) (0.0001s)
### EM.run with epoll ###
### EM started ###
HTTP.get:   (0.2536s) (0.2070s) (0.2530s) (0.2028s) (0.2031s)
redis.ping: (0.0507s) (0.0003s) (0.0505s) (0.0002s) (0.0503s)
### EM stopped ###
HTTP.get:   (0.0601s) (0.0778s) (0.0769s) (0.0938s) (0.0918s)
redis.ping: (0.0004s) (0.0001s) (0.0001s) (0.0001s) (0.0001s)

with-epoll-output-ree-1.8.7-2011.03.txt

### Before EM.run ###
HTTP.get:   (0.0561s) (0.0960s) (0.0815s) (0.0810s) (0.0600s)
redis.ping: (0.0009s) (0.0002s) (0.0003s) (0.0002s) (0.0001s)
### EM.run with epoll ###
### EM started ###
HTTP.get:   (0.6179s) (0.6203s) (0.6190s) (0.6714s) (0.6235s)
redis.ping: (0.1006s) (0.1005s) (0.1005s) (0.1006s) (0.1006s)
### EM stopped ###
HTTP.get:   (0.1120s) (0.1733s) (0.0920s) (0.0953s) (0.0818s)
redis.ping: (0.0003s) (0.0001s) (0.0001s) (0.0001s) (0.0001s)

without-epoll-output-ree-1.8.7-2011.03.txt

### Before EM.run ###
HTTP.get:   (0.3604s) (0.0806s) (0.0976s) (0.0842s) (0.0824s)
redis.ping: (0.0012s) (0.0003s) (0.0001s) (0.0001s) (0.0001s)
### EM.run without epoll ###
### EM started ###
HTTP.get:   (0.0910s) (0.0995s) (0.1046s) (0.0782s) (0.0784s)
redis.ping: (0.0003s) (0.0001s) (0.0001s) (0.0001s) (0.0001s)
### EM stopped ###
HTTP.get:   (0.0838s) (0.0948s) (0.0768s) (0.0676s) (0.0994s)
redis.ping: (0.0003s) (0.0001s) (0.0001s) (0.0001s) (0.0001s)

n.b. I tested this with ruby 1.9.3-p194, and the problem doesn't exist in that version anymore.