Merge pull request #4894 from Growstuff/puma-single-mode

Run Puma in single mode so the dyno stays under its memory limit

config/puma.rb

@@ -27,14 +27,35 @@ pidfile ENV.fetch("PIDFILE") { "tmp/pids/server.pid" }
 # Workers do not work on JRuby or Windows (both of which do not support
 # processes).
 #
-workers ENV.fetch("WEB_CONCURRENCY") { 2 }
-
-# Use the `preload_app!` method when specifying a `workers` number.
-# This directive tells Puma to first boot the application and load code
-# before forking the application. This takes advantage of Copy On Write
-# process behavior so workers use less memory.
+# MEMORY BUDGET: production is a 512 MB Heroku Basic dyno. Heroku raises R14 at
+# 512 MB and kills the dyno at ~1 GB (R15), so aim to keep the whole dyno below
+# 500 MB. Heroku sums the RSS of every process in the dyno, and each Ruby
+# process running this app costs roughly 200-300 MB, so the number of
+# processes is the main lever:
 #
-preload_app!
+#   * Clustered mode with a single worker runs a master that holds a full copy
+#     of the preloaded app next to the worker, and both are counted. That was
+#     ~620 MB, permanently over quota.
+#   * So 0 or 1 means single mode: one process, no master. Concurrency comes
+#     from the threads (RAILS_MAX_THREADS), which are cheap compared to
+#     processes.
+#   * Two or more workers will not fit in 512 MB. Only raise WEB_CONCURRENCY
+#     above 1 after moving to a bigger dyno, and check `heroku logs` for R14
+#     afterwards.
+#
+web_concurrency = Integer(ENV.fetch("WEB_CONCURRENCY") { 0 })
+
+# Puma reads WEB_CONCURRENCY itself, so single mode has to be set explicitly.
+workers(web_concurrency > 1 ? web_concurrency : 0)
+
+if web_concurrency > 1
+  # Use the `preload_app!` method when specifying a `workers` number.
+  # This directive tells Puma to first boot the application and load code
+  # before forking the application. This takes advantage of Copy On Write
+  # process behavior so workers use less memory.
+  #
+  preload_app!
+end
 
 # Allow puma to be restarted by `rails restart` command.
 plugin :tmp_restart