title: Why would an empty rails process have different memory footprint each time it starts up?
body: <p>We've been trying to figure out how to reduce the boot-up memory footprint of our rails app by identifying memory-hungry gems and finding alternatives or solutions.</p>
  
  <p>But there's one behavior on OS X that I find baffling.</p>
  
  <p>With a brand new generated rails app (<code>rails new memoryusage</code>), with no Gemfile, no models, no data and no transactions, upon starting up <code>rails c</code> the memory OSX displays for the corresponding ruby process vill vary each time it is started up, from as low as 60MB to as high as 65MB, with no discernible pattern as to why the same app might be requiring less or more memory per execution. </p>
  
  <p>I imagine this has to do in some way with how Ruby allocates memory, but I'm not completely clear on why its memory allocation would vary so wildly for the same code and no variable processing.</p>
  
  <p>We have similarly unpredictable behavior when we try to calculate the memory consumed by the process after each gem in the Gemfile is required. We load up a vanilla rails process, then in <code>rails c</code> we run a script that parses the Gemfile and requires each Gem individually, logging the memory pre- and post-require, and what we notice is that not only does the memory footprint not have a consistent starting point, but also the incremental 'steps' in our memory consumption vary wildly.</p>
  
  <p>We booted up our process three times, one after the other, and measured the start up memory and the incremental memory required by each gem. Not only did the startup book memory footprints bounce between 60MB and <strong><em>92MB</em></strong>, but the points at which we saw memory jumps on loading each gem were inconsistent -- sometimes loading SASS would eat up an additional 5MB, sometimes it wouldn't, sometimes active_merchant would demand 10MB additional, others it wouldn't.</p>
  
  <pre><code>                    :    BOOT UP #1            :    BOOT UP #2            :    BOOT UP #3           
  gem                 :  increment |      total  :  increment |      total  :  increment |      total
  rails               :       0.00 |      59.71  :       0.00 |      92.54  :       0.18 |      67.76
  unicorn             :       0.52 |      60.24  :       0.52 |      93.06  :       3.35 |      71.12
  haml                :       8.77 |      69.02  :       1.88 |      94.94  :       9.45 |      80.57
  haml-rails          :       0.00 |      69.02  :       0.00 |      94.94  :       0.00 |      80.57
  sass                :       4.36 |      73.38  :       6.95 |     101.89  :       0.99 |      81.55
  mongoid             :       0.00 |      73.38  :       0.00 |     101.89  :       0.00 |      81.55
  compass             :      11.56 |      84.93  :       3.23 |     105.12  :       8.41 |      89.96
  compass-rails       :       0.00 |      84.93  :       0.08 |     105.20  :       0.00 |      89.96
  compass_twitter_bootstrap:       0.00 |      84.93  :       0.00 |     105.20  :       0.00 |      89.96
  profanalyzer        :       0.59 |      85.52  :       0.46 |     105.66  :       0.64 |      90.60
  simple_form         :       0.34 |      85.87  :       0.35 |     106.01  :       0.00 |      90.60
  sorcery             :       0.00 |      85.87  :       0.25 |     106.26  :       1.07 |      91.67
  validates_timeliness:       1.47 |      87.34  :       1.82 |     108.07  :       1.62 |      93.29
  mongoid_token       :       0.00 |      87.34  :       0.00 |     108.07  :       0.00 |      93.29
  nested_form         :       0.00 |      87.34  :       0.00 |     108.07  :       0.01 |      93.30
  nokogiri            :       0.86 |      88.20  :       1.16 |     109.24  :       1.37 |      94.67
  carmen              :       0.00 |      88.20  :       0.07 |     109.30  :       0.00 |      94.67
  carrierwave/mongoid :       2.78 |      90.98  :       0.38 |     109.69  :       0.13 |      94.80
  yajl                :       0.04 |      91.02  :       0.04 |     109.73  :       0.04 |      94.84
  multi_json          :       0.00 |      91.02  :       0.00 |     109.73  :       0.00 |      94.84
  uuid                :       0.00 |      91.03  :       0.00 |     109.73  :       0.41 |      95.25
  tilt                :       0.00 |      91.03  :       0.00 |     109.73  :       0.00 |      95.25
  dynamic_form        :       0.00 |      91.04  :       0.00 |     109.73  :       0.00 |      95.25
  forem               :       0.03 |      91.07  :       0.00 |     109.73  :       0.00 |      95.25
  browser             :       0.00 |      91.07  :       0.00 |     109.73  :       0.00 |      95.25
  activemerchant      :       2.17 |      93.24  :       1.18 |     110.92  :      10.58 |     105.83
  kaminari            :       0.00 |      93.24  :       0.00 |     110.92  :       0.00 |     105.83
  merit               :       0.00 |      93.24  :       0.00 |     110.92  :       0.00 |     105.83
  memcachier          :       0.00 |      93.24  :       0.00 |     110.92  :       0.00 |     105.83
  dalli               :       0.01 |      93.25  :       0.05 |     110.96  :       0.34 |     106.17
  bitly               :       2.47 |      95.72  :       9.43 |     120.40  :       1.53 |     107.70
  em-synchrony        :       1.00 |      96.72  :       0.18 |     120.57  :       0.55 |     108.24
  em-http-request     :       5.56 |     102.28  :       2.15 |     122.72  :       1.40 |     109.64
  httparty            :       0.00 |     102.28  :       0.00 |     122.72  :       0.00 |     109.64
  rack-block          :       0.00 |     102.28  :       0.00 |     122.72  :       0.00 |     109.64
  resque/server       :       1.21 |     103.49  :       1.73 |     124.45  :       1.68 |     111.32
  resque_mailer       :       0.00 |     103.49  :       0.00 |     124.45  :       0.00 |     111.32
  rack-timeout        :       0.00 |     103.49  :       0.00 |     124.45  :       0.00 |     111.32
  chronic             :       1.66 |     105.15  :       0.67 |     125.12  :       0.64 |     111.96
  oink                :       0.00 |     105.15  :       0.00 |     125.12  :       0.00 |     111.96
  dotenv-rails        :       0.00 |     105.15  :       0.00 |     125.12  :       0.00 |     111.96
  jquery-rails        :       0.00 |     105.15  :       0.03 |     125.15  :       0.00 |     111.96
  jquery-ui-rails     :       0.00 |     105.15  :       0.00 |     125.15  :       0.00 |     111.96
  </code></pre>
  
  <p>It's clear to me that there's something very basic that I'm missing and don't understand about how memory is allocated to Ruby processes, but I'm having a hard time figuring out why it could be this seemingly stochastic. Anyone have any thoughts?</p>