Add scripts to collect GC setting results

This is useful to explore the impact of RUBY_GC_*
settings on application start-up performance and
GC stats.

doc/development/profiling.md

@@ -130,6 +130,8 @@ As a follow up to finding `N+1` queries with Bullet, consider writing a [QueryRe
 
 ## Settings that impact performance
 
+### Application settings
+
 1. `development` environment by default works with hot-reloading enabled, this makes Rails to check file changes every request, and create a potential contention lock, as hot reload is single threaded.
 1. `development` environment can load code lazily once the request is fired which results in first request to always be slow.
 
@@ -140,3 +142,34 @@ To disable those features for profiling/benchmarking set the `RAILS_PROFILE` env
 - restart GDK with `gdk restart`
 
 *This environment variable is only applicable for the development mode.*
+
+### GC settings
+
+Ruby's garbage collector (GC) can be tuned via a variety of environment variables that will directly impact application performance.
+
+The following table lists these variables along with their default values.
+
+| Environment variable | Default value |
+|--|--|
+| `RUBY_GC_HEAP_INIT_SLOTS` | `10000` |
+| `RUBY_GC_HEAP_FREE_SLOTS` | `4096` |
+| `RUBY_GC_HEAP_FREE_SLOTS_MIN_RATIO` | `0.20` |
+| `RUBY_GC_HEAP_FREE_SLOTS_GOAL_RATIO` | `0.40` |
+| `RUBY_GC_HEAP_FREE_SLOTS_MAX_RATIO` | `0.65` |
+| `RUBY_GC_HEAP_GROWTH_FACTOR` | `1.8` |
+| `RUBY_GC_HEAP_GROWTH_MAX_SLOTS` | `0 (disable)` |
+| `RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR` | `2.0` |
+| `RUBY_GC_MALLOC_LIMIT(_MIN)` | `(16 * 1024 * 1024 /* 16MB */)` |
+| `RUBY_GC_MALLOC_LIMIT_MAX` | `(32 * 1024 * 1024 /* 32MB */)` |
+| `RUBY_GC_MALLOC_LIMIT_GROWTH_FACTOR` | `1.4` |
+| `RUBY_GC_OLDMALLOC_LIMIT(_MIN)` | `(16 * 1024 * 1024 /* 16MB */)` |
+| `RUBY_GC_OLDMALLOC_LIMIT_MAX` | `(128 * 1024 * 1024 /* 128MB */)` |
+| `RUBY_GC_OLDMALLOC_LIMIT_GROWTH_FACTOR` | `1.2` |
+
+([Source](https://github.com/ruby/ruby/blob/45b29754cfba8435bc4980a87cd0d32c648f8a2e/gc.c#L254-L308))
+
+GitLab may decide to change these settings in order to speed up application performance, lower memory requirements, or both.
+
+You can see how each of these settings affect GC performance, memory use and application start-up time for an idle instance of
+GitLab by runnning the `scripts/perf/gc/collect_gc_stats.rb` script. It will output GC stats and general timing data to standard
+out as CSV.

scripts/perf/gc/collect_gc_stats.rb

+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+####
+# Loads GitLab application classes with a variety of GC settings and prints
+# GC stats and timing data to standard out as CSV.
+#
+# The degree of parallelism can be increased by setting the PAR environment
+# variable (default: 2).
+
+require 'benchmark'
+
+SETTINGS = {
+  'DEFAULTS' => [''],
+  # Default: 10_000
+  'RUBY_GC_HEAP_INIT_SLOTS' => %w[100000 1000000 5000000],
+  # Default: 1.8
+  'RUBY_GC_HEAP_GROWTH_FACTOR' => %w[1.2 1 0.8],
+  # Default: 0 (disabled)
+  'RUBY_GC_HEAP_GROWTH_MAX_SLOTS' => %w[10000 100000 1000000],
+  # Default: 2.0
+  'RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR' => %w[2.5 1.5 1],
+  # Default: 4096 (= 2^12)
+  'RUBY_GC_HEAP_FREE_SLOTS' => %w[16384 2048 0],
+  # Default: 0.20 (20%)
+  'RUBY_GC_HEAP_FREE_SLOTS_MIN_RATIO' => %w[0.1 0.01 0.001],
+  # Default: 0.40 (40%)
+  'RUBY_GC_HEAP_FREE_SLOTS_GOAL_RATIO' => %w[0.2 0.01 0.001],
+  # Default: 0.65 (65%)
+  'RUBY_GC_HEAP_FREE_SLOTS_MAX_RATIO' => %w[0.2 0.02 0.002],
+  # Default: 16MB
+  'RUBY_GC_MALLOC_LIMIT' => %w[8388608 4194304 1048576],
+  # Default: 32MB
+  'RUBY_GC_MALLOC_LIMIT_MAX' => %w[16777216 8388608 1048576],
+  # Default: 1.4
+  'RUBY_GC_MALLOC_LIMIT_GROWTH_FACTOR' => %w[1.6 1 0.8],
+  # Default: 16MB
+  'RUBY_GC_OLDMALLOC_LIMIT' => %w[8388608 4194304 1048576],
+  # Default: 128MB
+  'RUBY_GC_OLDMALLOC_LIMIT_MAX' => %w[33554432 16777216 1048576],
+  # Default: 1.2
+  'RUBY_GC_OLDMALLOC_LIMIT_GROWTH_FACTOR' => %w[1.4 1 0.8]
+}.freeze
+
+USED_GCSTAT_KEYS = [
+  :minor_gc_count,
+  :major_gc_count,
+  :heap_live_slots,
+  :heap_free_slots,
+  :total_allocated_pages,
+  :total_freed_pages,
+  :malloc_increase_bytes,
+  :malloc_increase_bytes_limit,
+  :oldmalloc_increase_bytes,
+  :oldmalloc_increase_bytes_limit
+].freeze
+
+CSV_USED_GCSTAT_KEYS = USED_GCSTAT_KEYS.join(',')
+CSV_HEADER = "setting,value,#{CSV_USED_GCSTAT_KEYS},RSS,gc_time_s,cpu_utime_s,cpu_stime_s,real_time_s\n"
+
+SCRIPT_PATH = __dir__
+RAILS_ROOT = "#{SCRIPT_PATH}/../../../"
+
+def collect_stats(setting, value)
+  warn "Testing #{setting} = #{value} ..."
+  env = {
+    setting => value,
+    'RAILS_ROOT' => RAILS_ROOT,
+    'SETTING_CSV' => "#{setting},#{value}",
+    'GC_STAT_KEYS' => CSV_USED_GCSTAT_KEYS
+  }
+  system(env, 'ruby', "#{SCRIPT_PATH}/print_gc_stats.rb")
+end
+
+par = ENV['PAR']&.to_i || 2
+batch_size = (SETTINGS.size.to_f / par).ceil
+batches = SETTINGS.each_slice(batch_size)
+
+warn "Requested parallelism: #{par} (batches: #{batches.size}, batch size: #{batch_size})"
+
+puts CSV_HEADER
+
+elapsed = Benchmark.realtime do
+  threads = batches.each_with_index.map do |settings_batch, n|
+    Thread.new do
+      settings_batch.each do |setting, values|
+        values.each do |v|
+          collect_stats(setting, v)
+        end
+      end
+    end
+  end
+
+  threads.each(&:join)
+end
+
+warn "All done in #{elapsed} sec"

scripts/perf/gc/print_gc_stats.rb

+# frozen_string_literal: true
+
+# Promotes survivors from eden to old gen and runs a compaction.
+#
+# aka "Nakayoshi GC"
+#
+# https://github.com/puma/puma/blob/de632261ac45d7dd85230c83f6af6dd720f1cbd9/lib/puma/util.rb#L26-L35
+def nakayoshi_gc
+  4.times { GC.start(full_mark: false) }
+  GC.compact
+end
+
+# GC::Profiler is used elsewhere in the code base, so we provide a way for it
+# to be used exclusively by this script, or otherwise results will be tainted.
+module GC::Profiler
+  class << self
+    attr_accessor :use_exclusive
+
+    %i[enable disable clear].each do |method|
+      alias_method "#{method}_orig", "#{method}"
+
+      define_method(method) do
+        if use_exclusive
+          warn "GC::Profiler: ignoring call to #{method}"
+          return
+        end
+
+        send("#{method}_orig") # rubocop: disable GitlabSecurity/PublicSend
+      end
+    end
+  end
+end
+
+GC::Profiler.enable
+GC::Profiler.use_exclusive = true
+
+require 'benchmark'
+
+RAILS_ROOT = ENV['RAILS_ROOT']
+
+tms = Benchmark.measure do
+  require RAILS_ROOT + 'config/boot'
+  require RAILS_ROOT + 'config/environment'
+end
+
+GC::Profiler.use_exclusive = false
+
+nakayoshi_gc
+
+gc_stats = GC.stat
+warn gc_stats.inspect
+
+gc_total_time = GC::Profiler.total_time
+
+GC::Profiler.report($stderr)
+GC::Profiler.disable
+
+gc_stat_keys = ENV['GC_STAT_KEYS'].to_s.split(',').map(&:to_sym)
+
+values = []
+values << ENV['SETTING_CSV']
+values += gc_stat_keys.map { |k| gc_stats[k] }
+values << ::Gitlab::Metrics::System.memory_usage_rss
+values << gc_total_time
+values << tms.utime + tms.cutime
+values << tms.stime + tms.cstime
+values << tms.real
+
+puts values.join(',')