Estimating the per-entry overhead of ActiveSupport::Cache::MemoryStore.

cache_size.rake

task :cache_size => :environment do
  require 'objspace'

  def calc_overhead(run, count)
    cache = ActiveSupport::Cache.lookup_store :memory_store, size: 128.gigabytes
    # keep all key/value pairs unique
    cache.instance_variable_get(:'@data').compare_by_identity

    GC.start
    before = ObjectSpace.count_objects_size({})[:TOTAL]

    GC.start
    count.times { cache.fetch(Object.new) { Object.new } }

    GC.start
    after = ObjectSpace.count_objects_size({})[:TOTAL]

    entry_size = ActiveSupport::Cache::Entry.new(Object.new).size
    overhead = 1.0 * (after - before) / count - entry_size
    printf "%3d   %12d   %12d   %8d   %s\n", run, before, after, overhead, cache.inspect
    overhead
  end

  puts "Estimating ActiveSupport::Cache::MemoryStore overhead on #{Time.now}"
  p [RUBY_ENGINE, RUBY_VERSION, RUBY_PATCHLEVEL, RUBY_PLATFORM]

  runs, count = 10, 2**16
  puts "\n#{runs} runs of #{count} #fetch operations:\n\n"
  puts "Run     Before          After       Overhead   Cache\n"
  puts "---     ----------      ---------   --------   -----\n"
  avg_overhead = (1..runs).map { |i| calc_overhead(i, count) }.sum / runs
  puts "\n---> Average overhead for #{runs} runs with size #{count} is #{avg_overhead.floor}"
end

output_ruby-1.9.3p429-macosx

Estimating ActiveSupport::Cache::MemoryStore overhead on 2013-07-20 21:51:00 -0400
["ruby", "1.9.3", 429, "x86_64-darwin12.4.0"]

10 runs of 65536 #fetch operations:

Run     Before          After       Overhead   Cache
---     ----------      ---------   --------   -----
  1       79707791       96374071        242   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  2       79707911       96418223        242   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  3       79707911       96389887        242   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  4       79707911       96394735        242   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  5       79708063       96392007        242   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  6       79708063       96385047        242   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  7       79708063       96394887        242   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  8       79708063       96369063        242   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  9       79708063       96400647        242   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
 10       79708063       96392007        242   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>

---> Average overhead for 10 runs with size 65536 is 242

output_ruby-2.0.0p247-macosx

Estimating ActiveSupport::Cache::MemoryStore overhead on 2013-07-20 21:45:33 -0400
["ruby", "2.0.0", 247, "x86_64-darwin12.4.0"]

10 runs of 65536 #fetch operations:

Run     Before          After       Overhead   Cache
---     ----------      ---------   --------   -----
  1       85518335      102035095        240   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  2       85518639      102039943        240   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  3       85518639      102042775        240   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  4       85518639      102039895        240   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  5       85518799      102040247        240   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  6       85518799      102038183        240   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  7       85518799      102041495        240   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  8       85518799      102039671        240   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
  9       85518799      102040823        240   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>
 10       85518799      102032135        239   <#ActiveSupport::Cache::MemoryStore entries=65536, size=786432, options={:size=>137438953472}>

---> Average overhead for 10 runs with size 65536 is 240