Determine Dyno RAM (#1789)

If $DYNO_RAM is set, us that, otherwise use the cgroup memory
controller's limit info to calculate the RAM limit. Failing either of
those return the default of 512.

This works in Heroku's Dyno containers and may/probably works in other
containers that mount in the containers cgroup controler.

config/initializers/puma_worker_killer.rb

@@ -1,7 +1,19 @@
 # frozen_string_literal: true
+
+# return a dyno's ram limit in MB based on: $DYNO_RAM,
+# /sys/fs/cgroup/memory/memory.limit_in_bytes, or the default of 512
+def dyno_ram
+  if !ENV['DYNO_RAM'].nil?
+    return Integer(ENV['DYNO_RAM'])
+  end
+  Integer(Integer(IO.read("/sys/fs/cgroup/memory/memory.limit_in_bytes")) / 1024 / 1024)
+  rescue
+    return 512
+end
+
 if Rails.env.production?
   PumaWorkerKiller.config do |config|
-    config.ram           = Integer(ENV['DYNO_RAM'] || 512) # mb
+    config.ram           = dyno_ram
     config.frequency     = 5    # seconds
     config.percent_usage = 0.98
     config.rolling_restart_frequency = 6 * 3600 # 6 hours in seconds