Help me memory problem

OS: Windows7 32bit
main memory : 4GB
ruby -v : ruby 1.8.6 (2008-08-11 patchlevel 287) [i386-mswin32]

big.mkv file size : 1.45GB

ex1.rb

puts $$

File.open(“D:/test/big.mkv”, “rb”) do |f|
while buff = f.read(4096)
end
end

sleep 1000

ex1.rb is OK!! memory usages is about 19,756 KB.

But…

ex2.rb

puts $$

th1 = Thread.new do
loop do
sleep 1
end
end

File.open(“D:/test/big.mkv”, “rb”) do |f|
while buff = f.read(4096)
end
end

th1.join

ex2.rb
memory usages is increased continually… after all 1,937,948 KB

I have to use Thread…
Please… Help Me!!