wrapper script for starting ruby with configured jemalloc 5.md

1 intro.md

invoke with bin/jemalloc rails s

We weren’t using 3 before, but have been running 5 in prod for ~10 months now, we saw ~25% less memory usage compared to default malloc

see also https://gist.github.com/jjb/9ff0d3f622c8bbe904fe7a82e35152fc

2 code.rb

#!/usr/bin/env ruby
# frozen_string_literal: true

ENV['LD_PRELOAD'] = ENV['JEMALLOC_PATH']
ENV['MALLOC_CONF'] = {
  # Maximum number of arenas to use for automatic multiplexing of threads and arenas.
  # The default is four times the number of CPUs, or one if there is a single CPU.
  narenas: 2,

  # Enabling jemalloc background threads generally improves the tail latency for application threads,
  # since unused memory purging is shifted to the dedicated background threads.
  # In addition, unintended purging delay caused by application inactivity is avoided with background threads.
  #
  # Suggested: background_thread:true when jemalloc managed threads can be allowed.
  background_thread: true,

  # Transparent hugepage (THP) mode.
  # Settings "always", "never" and "default" are available if THP is supported by the operating system.
  # The "always" setting enables transparent hugepage for all user memory mappings with MADV_HUGEPAGE;
  # "never" ensures no transparent hugepage with MADV_NOHUGEPAGE; the default setting "default" makes no changes.
  thp: 'never',

  # Decay time determines how fast jemalloc returns unused pages back to the operating system,
  # and therefore provides a fairly straightforward trade-off between CPU and memory usage.
  # Shorter decay time purges unused pages faster to reduces memory usage
  # (usually at the cost of more CPU cycles spent on purging), and vice versa.
  #
  # Suggested: tune the values based on the desired trade-offs.
  dirty_decay_ms: 1_000, # Default 10_000
  muzzy_decay_ms: 0 # Default 10_000
}.map { |conf| conf.join(':') }.join(',').freeze # Convert to a MALLOC_CONF string

require 'pathname'
ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../Gemfile',
                                           Pathname.new(__FILE__).realpath)

bundle_binstub = File.expand_path('bundle', __dir__)

if File.file?(bundle_binstub)
  if /This file was generated by Bundler/.match?(File.read(bundle_binstub, 300))
    load(bundle_binstub)
  else
    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
  end
end

require 'rubygems'
require 'bundler/setup'

arg = ARGV.shift
if arg.nil?
  warn "Usage: #{$PROGRAM_NAME} executable_name"
  warn "Usage: #{$PROGRAM_NAME} gem_name/executable_name"
  exit(1)
end

# If the executable is the same as the gem, we can pass "executable_name" vs "gem_name/executable_name"
gem, executable = arg.split('/')
executable ||= gem
gem = 'railties' if gem == 'rails' # Rails has no executables, they're stored in railties

if gem.empty?
  warn('Exiting: Must supply a gem bin or gem_name/executable')
  exit(1)
end

load Gem.bin_path(gem, executable)