diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3e645edf7..196124535 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,7 @@
 ## v205 (unreleased)
 
+* Default `MALLOC_ARENA_MAX=2` for new applications (https://github.com/heroku/heroku-buildpack-ruby/pull/752)
+
 ## v204 (9/12/2019)
 
 * Default Ruby version for new apps is now 2.5.6 (https://github.com/heroku/heroku-buildpack-ruby/pull/919)
diff --git a/changelogs/v205/malloc.md b/changelogs/v205/malloc.md
new file mode 100644
index 000000000..b8cd03e8e
--- /dev/null
+++ b/changelogs/v205/malloc.md
@@ -0,0 +1,32 @@
+## Default `MALLOC_ARENA_MAX=2` for new Ruby applications
+
+The environment variable `MALLOC_ARENA_MAX` will now default to `2` for Ruby applications. This environment variable was
+previously unset. This change will only affect new applications on the platform, to update an existing application please
+run:
+
+```
+$ heroku config:set MALLOC_ARENA_MAX=2
+```
+
+The goal of setting this value is to decrease memory usage for the majority of Ruby applications that are using threads
+such as apps that use Sidekiq or Puma. To understand more about the relationship between this value and memory please see
+the following external resources:
+
+- https://www.speedshop.co/2017/12/04/malloc-doubles-ruby-memory.html
+- https://www.mikeperham.com/2018/04/25/taming-rails-memory-bloat/
+
+We also maintain our own [documentation on tuning the memory behavior of glibc by setting this environment variable](https://devcenter.heroku.com/articles/tuning-glibc-memory-behavior).
+
+If a your application is not memory bound and would prefer slightly faster execution over the decreased memory use,
+you can set their `MALLOC_ARENA_MAX` to a higher value. The default as specified by the [linux man page](http://man7.org/linux/man-pages/man3/mallopt.3.html)
+is 8 times the number of cores on the system.
+
+## Jemalloc
+
+Another popular alternative memory allocator is jemalloc. At this time Heroku does not maintain a supported version of this memory allocator,
+but you can use it in your application with a 3rd party [jemalloc buildpack](https://elements.heroku.com/buildpacks/mojodna/heroku-buildpack-jemalloc).
+
+If you are using jemalloc, setting `MALLOC_ARENA_MAX` will have no impact on memory or performance. For more information on
+how jemalloc interacts with Ruby applications see this external post:
+
+- https://www.speedshop.co/2017/12/04/malloc-doubles-ruby-memory.html#fix-2-use-jemalloc
diff --git a/lib/language_pack/metadata.rb b/lib/language_pack/metadata.rb
index f6979883c..4cc1d33e1 100644
--- a/lib/language_pack/metadata.rb
+++ b/lib/language_pack/metadata.rb
@@ -13,7 +13,7 @@ def initialize(cache)
 
   def read(key)
     full_key = "#{FOLDER}/#{key}"
-    File.read(full_key) if exists?(key)
+    File.read(full_key).chomp if exists?(key)
   end
 
   def exists?(key)
@@ -27,6 +27,12 @@ def write(key, value, isave = true)
     full_key = "#{FOLDER}/#{key}"
     File.open(full_key, 'w') {|f| f.puts value }
     save if isave
+
+    return true
+  end
+
+  def touch(key)
+    write(key, "true")
   end
 
   def save
diff --git a/lib/language_pack/ruby.rb b/lib/language_pack/ruby.rb
index ee72e3c50..8de8513cc 100644
--- a/lib/language_pack/ruby.rb
+++ b/lib/language_pack/ruby.rb
@@ -121,6 +121,22 @@ def config_detect
 
 private
 
+  def default_malloc_arena_max?
+    return true if @metadata.exists?("default_malloc_arena_max")
+    return @metadata.touch("default_malloc_arena_max") if new_app?
+
+    return false
+  end
+
+  def stack_not_14_not_16?
+    case stack
+    when "cedar-14", "heroku-16"
+      return false
+    else
+      return true
+    end
+  end
+
   def warn_bundler_upgrade
     old_bundler_version  = @metadata.read("bundler_version").chomp if @metadata.exists?("bundler_version")
 
@@ -352,6 +368,7 @@ def setup_profiled
       set_env_override "GEM_PATH", "$HOME/#{slug_vendor_base}:$GEM_PATH"
       set_env_override "PATH",      profiled_path.join(":")
 
+      set_env_default "MALLOC_ARENA_MAX", "2"     if default_malloc_arena_max?
       add_to_profiled set_default_web_concurrency if env("SENSIBLE_DEFAULTS")
 
       if ruby_version.jruby?
@@ -568,11 +585,7 @@ def load_default_cache
   # install libyaml into the LP to be referenced for psych compilation
   # @param [String] tmpdir to store the libyaml files
   def install_libyaml(dir)
-    case stack
-    when "cedar-14", "heroku-16"
-    else
-      return
-    end
+    return false if stack_not_14_not_16?
 
     instrument 'ruby.install_libyaml' do
       FileUtils.mkdir_p dir
diff --git a/spec/hatchet/rubies_spec.rb b/spec/hatchet/rubies_spec.rb
index 283d97138..0cfa68755 100644
--- a/spec/hatchet/rubies_spec.rb
+++ b/spec/hatchet/rubies_spec.rb
@@ -85,6 +85,8 @@
     app = Hatchet::Runner.new("default_ruby", stack: DEFAULT_STACK)
     app.setup!
     app.deploy do |app|
+      expect(app.run('echo "MALLOC_ARENA_MAX_is=$MALLOC_ARENA_MAX"')).to match("MALLOC_ARENA_MAX_is=2")
+
       run!(%Q{echo "ruby '2.5.1'" >> Gemfile})
       run!("git add -A; git commit -m update-ruby")
       app.push!
@@ -93,4 +95,4 @@
       expect(app.output).to match("Ruby version change detected")
     end
   end
-end
\ No newline at end of file
+end