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@mperham mperham/frag.rb
Last active Jun 4, 2019

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memory fragmentation on ruby 2.5.1
=begin
This script attempts to reproduce poor glibc allocator behavior within Ruby, leading
to extreme memory fragmentation and process RSS bloat.
glibc allocates memory using per-thread "arenas". These blocks can easily fragment when
some objects are free'd and others are long-lived.
Our script runs multiple threads, all allocating randomly sized "large" Strings between 4,000
and 40,000 bytes in size. This simulates Rails views with ERB creating large chunks of HTML
to output to the browser. Some of these strings are kept around and some are discarded.
With the builds below and the frag.rb script, jemalloc and MALLOC_ARENA_MAX=2 both show a noticeable reduction in RSS.
=end
=begin
# This script is run on a 4-core 8GB instance.
# system ruby is only necessary to compile our own ruby
#
# with a blank ubuntu 18.04 image...
apt-get update
apt-get install -y autoconf bison build-essential libssl-dev libyaml-dev libreadline-dev zlib1g-dev libncurses5-dev libffi-dev libgdbm-compat-dev libgdbm-dev libjemalloc-dev ruby
curl -LO https://cache.ruby-lang.org/pub/ruby/2.5/ruby-2.5.1.tar.bz2
tar xvf ruby-2.5.1.tar.bz2
cd ruby-2.5.1
./configure --disable-install-doc --prefix=/root/versions/2.5.1
make -j4
make install
make distclean
./configure --with-jemalloc --disable-install-doc --prefix=/root/versions/2.5.1j
make -j4
make install
/root/versions/2.5.1/bin/ruby -v -rrbconfig -e 'p RbConfig::CONFIG["configure_args"]'
/root/versions/2.5.1j/bin/ruby -v -rrbconfig -e 'p RbConfig::CONFIG["configure_args"]'
##### RESULTS
# note on a 4-core machine, MALLOC_ARENA_MAX defaults to (cores * 8) so we are merely being explicit here.
#
> MALLOC_ARENA_MAX=32 /root/versions/2.5.1/bin/ruby -v frag.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
'--disable-install-doc' '--prefix=/root/versions/2.5.1'
Total string size: 1903MB
VmRSS: 2831832 kB
> MALLOC_ARENA_MAX=2 /root/versions/2.5.1/bin/ruby -v frag.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
'--disable-install-doc' '--prefix=/root/versions/2.5.1'
Total string size: 1917MB
VmRSS: 2311052 kB
> /root/versions/2.5.1j/bin/ruby -v frag.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
'--with-jemalloc' '--disable-install-doc' '--prefix=/root/versions/2.5.1j'
Total string size: 1908MB
VmRSS: 2306372 kB
=end
Thread.abort_on_exception = true
require 'rbconfig'
puts RbConfig::CONFIG["configure_args"]
THREAD_COUNT = (ENV["T"] || 10).to_i
Threads = []
Strings = []
Threads.clear
srand(1234)
def frag
saver = []
# requests
100.times do |idx|
# each request allocates 1000 strings, then discards 90%
1_000.times do |x|
# allocate a random sized heap string
s = 'a' * ((rand(4000) * 10) + 97)
saver << s
end
# now delete random elements to create holes in the heap
1000.times do
saver.delete_at(rand(saver.size)) if rand < 0.9
end
GC.start if idx % 100 == 0
end
total = saver.inject(0) {|memo, str| memo += str.bytesize }
#print "Thread #{Thread.current.object_id.to_s(36)} has #{total / (1024 * 1024)}MB\n"
Strings << saver
total
end
THREAD_COUNT.times do
Threads << Thread.new(&method(:frag))
end
strsize = 0
Threads.each do|t|
strsize += t.value
end
Threads.clear
GC.start
puts "Total string size: #{strsize / (1024 * 1024)}MB"
puts `ps aux | grep #{$$}` if RUBY_PLATFORM =~ /darwin/
IO.foreach("/proc/#{$$}/status") do |line|
print line if line =~ /VmRSS/
end if RUBY_PLATFORM =~ /linux/
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