Reduce allocations in keys#read key by Antsiscool · Pull Request #766 · mongomapper/mongomapper

Antsiscool · 2026-10-26T03:03:33Z

Ruby 3.0 added Symbol#name which returns an interned string. This reduces allocations.

master     :title        0.349s      1000002 allocations (1.00 per call)
master     "title"       0.338s            0 allocations (0.00 per call)
current    :title        0.304s            2 allocations (0.00 per call)
current    "title"       0.322s            0 allocations (0.00 per call)

master     :title        0.349s      1000002 allocations (1.00 per call)
master     "title"       0.337s            0 allocations (0.00 per call)
current    :title        0.299s            2 allocations (0.00 per call)
current    "title"       0.321s            0 allocations (0.00 per call)

Here is the benchmark code:

# Benchmarks MongoMapper::Plugins::Keys#read_key on the current checkout
# against master (or any git ref passed as the first argument).
#
#   ruby performance/read_key.rb          # compare against master
#   ruby performance/read_key.rb v0.17.0  # compare against another ref
#
# No database connection is needed; documents are only built in memory.

require 'rbconfig'
require 'tmpdir'

ITERATIONS = 1_000_000
ROUNDS = 5

if ENV['MM_BENCH_LIB']
  $:.unshift ENV['MM_BENCH_LIB']
  require 'mongo_mapper'

  class BenchDoc
    include MongoMapper::Document
    key :title, String
    key :count, Integer
  end

  doc = BenchDoc.load('_id' => BSON::ObjectId.new, 'title' => 'Testing', 'count' => 1)

  # The key is created once, outside the loop, so a String literal
  # doesn't add an allocation of its own to every call.
  [:title, 'title'].each do |key_name|
    # warm up
    10_000.times { doc[key_name] }

    GC.start
    GC.disable
    before = GC.stat(:total_allocated_objects)
    ITERATIONS.times { doc[key_name] }
    allocations = GC.stat(:total_allocated_objects) - before
    GC.enable

    # best of ROUNDS to reduce noise from other processes
    real = ROUNDS.times.map do
      start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
      ITERATIONS.times { doc[key_name] }
      Process.clock_gettime(Process::CLOCK_MONOTONIC) - start
    end.min

    puts format('%-10s %-8s %10.3fs %12d allocations (%.2f per call)',
                ENV['MM_BENCH_LABEL'], key_name.inspect, real, allocations, allocations.to_f / ITERATIONS)
  end
else
  root = `git rev-parse --show-toplevel`.strip
  ref = ARGV[0] || 'master'

  Dir.mktmpdir do |dir|
    system("git -C #{root} archive #{ref} lib | tar -x -C #{dir}", exception: true)

    puts "ruby #{RUBY_VERSION}, #{ITERATIONS} calls to read_key, best of #{ROUNDS} rounds"
    [[ref, File.join(dir, 'lib')], ['current', File.join(root, 'lib')]].each do |label, lib|
      env = { 'MM_BENCH_LIB' => lib, 'MM_BENCH_LABEL' => label }
      system(env, RbConfig.ruby, __FILE__, exception: true)
    end
  end
end

Symbol#name returns an interned frozen string, whilst Symbol#to_s allocates a new string.

Antsiscool · 2026-11-03T10:51:03Z

@smtlaissezfaire This further reduces some allocations and improves the performance by storing ivar in a local variable.

Antsiscool added 2 commits October 26, 2026 12:39

Use Symbol#name when available in MongoMapper::Plugins::Keys#read_key

37652fa

Symbol#name returns an interned frozen string, whilst Symbol#to_s allocates a new string.

Reduce branching

cb37e69

smtlaissezfaire merged commit dbe9faf into mongomapper:master Nov 3, 2026
66 checks passed

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Reduce allocations in keys#read key - #766

Reduce allocations in keys#read key#766
smtlaissezfaire merged 2 commits into
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Antsiscool:reduce-allocations-in-keys#read_key

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