test to see if GC in ruby 2 is truly copy on write friendly

gc-cow.rb

#!/usr/bin/env ruby

rss = '.+?Rss:\s+(\d+)'
share =  '.+?Shared_Clean:\s+(\d+)'
share << '.+?Shared_Dirty:\s+(\d+)'
priv =  '.+?Private_Clean:\s+(\d+)'
priv << '.+?Private_Dirty:\s+(\d+)'
MEM_REGEXP = /\[heap\]#{rss}#{share}#{priv}/m

def mem_usage()
  if File.read("/proc/#{Process.pid}/smaps").match(MEM_REGEXP)
    rss = $1.to_i
    share = $2.to_i + $3.to_i
    priv = $4.to_i + $5.to_i
  end
  "RSS: #{rss} kB, Shared: #{share} kB, Private: #{priv} kB"
end

puts "parent (#{Process.pid}) baseline memory: #{mem_usage}"

alloc=10000 * 2 ** 10
puts "parent allocating #{alloc} objects"
a = []
alloc.times do
  a << Object.new
end

puts "parent allocated, mem usage: #{mem_usage}"

GC.start
GC.start
GC.start

puts "parent GC'd, mem usage: #{mem_usage}"
sleep 2

if fork
	puts "parent (#{Process.pid}) GC'd and sleeping, mem usage: #{mem_usage}"
	sleep 40
else
	puts "child (#{Process.pid}) forked, mem usage: #{mem_usage}"
	GC.start
	GC.start
	GC.start
	puts "child (#{Process.pid}) GC'd and sleeping, mem usage: #{mem_usage}"	
	sleep 30
end

test run of the above

% ruby -v
ruby 2.0.0dev (2012-03-23 trunk 35121) [x86_64-linux]
exit status: 0
% ruby gc-cow.rb
parent (16718) baseline memory: RSS: 2552 kB, Shared: 0 kB, Private: 2552 kB
parent allocating 10240000 objects
parent allocated, mem usage: RSS: 442784 kB, Shared: 0 kB, Private: 442784 kB
parent GC'd, mem usage: RSS: 446920 kB, Shared: 0 kB, Private: 446920 kB
parent (16718) GC'd and sleeping, mem usage: RSS: 446952 kB, Shared: 446908 kB, Private: 44 kB
child (16738) forked, mem usage: RSS: 446952 kB, Shared: 446908 kB, Private: 44 kB
child (16738) GC'd and sleeping, mem usage: RSS: 446976 kB, Shared: 442636 kB, Private: 4340 kB

---
% ruby -v
ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-linux]
% ruby gc-cow.rb
parent (17090) baseline memory: RSS: 2456 kB, Shared: 0 kB, Private: 2456 kB
parent allocating 10240000 objects
parent allocated, mem usage: RSS: 439196 kB, Shared: 0 kB, Private: 439196 kB
parent GC'd, mem usage: RSS: 439252 kB, Shared: 0 kB, Private: 439252 kB
child (17102) forked, mem usage: RSS: 439288 kB, Shared: 439252 kB, Private: 36 kB
parent (17090) GC'd and sleeping, mem usage: RSS: 439288 kB, Shared: 439252 kB, Private: 36 kB
child (17102) GC'd and sleeping, mem usage: RSS: 439312 kB, Shared: 1564 kB, Private: 437748 kB