From 33e9aa45bedd542398a872f19383ada707fb2bf9 Mon Sep 17 00:00:00 2001
From: Thomas Klemm <github@tklemm.eu>
Date: Thu, 5 Nov 2026 18:43:24 +0000
Subject: [PATCH] Run lobsters with a fixed clock

Lobsters' work per iteration changed with the calendar date: in its US Central time zone, lookbacks such as 70.days.ago that cross a DST change make ActiveSupport look up the zone period again, and two dates switch the logo. Time.now now returns a fixed instant in July, from which no lookback (up to 90 days) crosses a DST change. Harness code outside the benchmark block, including harnesses that HARNESS=chain loads late, keeps the real clock. TRACK_AR_TIME measures with the monotonic clock.
---
 benchmarks/lobsters/benchmark.rb | 51 ++++++++++++++++++++------------
 1 file changed, 32 insertions(+), 19 deletions(-)

diff --git a/benchmarks/lobsters/benchmark.rb b/benchmarks/lobsters/benchmark.rb
index 9bf6c0e1..11b52d31 100644
--- a/benchmarks/lobsters/benchmark.rb
+++ b/benchmarks/lobsters/benchmark.rb
@@ -6,42 +6,53 @@
 Dir.chdir __dir__
 use_gemfile
 
-require_relative 'config/environment'
-require_relative "route_generator"
-
-# For an in-mem DB, we need to load all data on every boot
-mem_db = ActiveRecord::Base.connection.raw_connection
-file_db = SQLite3::Database.new('db/production.sqlite3')
-b = SQLite3::Backup.new(mem_db, 'main', file_db, 'main')
-b.step(-1) # import until finished
-b.finish # destroy the Backup object
-# The fixture has no planner statistics. Without them, SQLite misplans the
-# hottest-stories query behind /, /recent and /rss.
-mem_db.execute("ANALYZE")
-
-app = Rails.application
-generator = RouteGenerator.new(app)
-generator.routes # Make sure routes have been pregenerated
+# Avoid calendar-dependent DST and holiday-logo work.
+FROZEN_NOW = Time.utc(2026, 7, 1, 12, 0, 0, 123456.789r).getlocal.freeze
+REAL_NOW = Time.method(:now)
+def Time.now = $clock_frozen ? FROZEN_NOW.dup : REAL_NOW.call
+
+begin
+  $clock_frozen = true
+  require_relative 'config/environment'
+  require_relative "route_generator"
+
+  # For an in-mem DB, we need to load all data on every boot
+  mem_db = ActiveRecord::Base.connection.raw_connection
+  file_db = SQLite3::Database.new('db/production.sqlite3')
+  b = SQLite3::Backup.new(mem_db, 'main', file_db, 'main')
+  b.step(-1) # import until finished
+  b.finish # destroy the Backup object
+  # The fixture has no planner statistics. Without them, SQLite misplans the
+  # hottest-stories query behind /, /recent and /rss.
+  mem_db.execute("ANALYZE")
+
+  app = Rails.application
+  generator = RouteGenerator.new(app)
+  generator.routes # Make sure routes have been pregenerated
+ensure
+  $clock_frozen = false
+end
 
 # Track ActiveRecord time
 if ENV['TRACK_AR_TIME']
   ar_total_duration = 0.0
-  process_start_t = Time.now
+  process_start_t = Process.clock_gettime(Process::CLOCK_MONOTONIC)
 
   # Track sql.active_record events
-  ActiveSupport::Notifications.subscribe "sql.active_record" do |name, started, finished, unique_id, data|
+  ActiveSupport::Notifications.monotonic_subscribe "sql.active_record" do |name, started, finished, unique_id, data|
     duration = finished - started
     ar_total_duration += duration
   end
 
   at_exit do
-    process_duration = Time.now - process_start_t
+    process_duration = Process.clock_gettime(Process::CLOCK_MONOTONIC) - process_start_t
     ar_percent = ar_total_duration * 100.0 / process_duration
     puts "ActiveRecord time: #{ar_total_duration.round(2)}s (#{ar_percent.round(2)}%) PID: #{Process.pid}"
   end
 end
 
 run_benchmark(10) do
+  $clock_frozen = true
   # Routes are from ./routes_generator.rb
   generator.routes.each_with_index do |env, idx|
     path = env["PATH_INFO"] # app.call mutates the path
@@ -52,6 +63,8 @@
     end
     response_array.last.close # Response might be a Rack::BodyProxy and MUST be closed.
   end
+ensure
+  $clock_frozen = false
 end
 
 # This benchmark will keep writing the production log on every request. It adds up.