unicorn out of band GC

Author: SamSaffron

config/initializers/99-unicorn.rb

@@ -1,4 +1,4 @@
-if (oobgc=ENV['UNICORN_OOBGC_REQS'].to_i) > 0
-   require 'unicorn/oob_gc'
-   Rails.configuration.middleware.insert 0, Unicorn::OobGC, oobgc
+if ENV['UNICORN_ENABLE_OOBGC']
+  require 'middleware/unicorn_oobgc'
+  Middleware::UnicornOobgc.init
 end

config/unicorn.conf.rb

@@ -1,7 +1,7 @@
 # See http://unicorn.bogomips.org/Unicorn/Configurator.html
 
-# RUN out of band GC every 2 requests
-# ENV['UNICORN_OOBGC_REQS'] = "2" unless ENV['UNICORN_OOBGC_REQS']
+# enable out of band gc out of the box, it is low risk and improves perf a lot
+ENV['UNICORN_ENABLE_OOBGC'] = "1"
 
 discourse_path = File.expand_path(File.expand_path(File.dirname(__FILE__)) + "/../")
 

lib/middleware/unicorn_oobgc.rb

@@ -0,0 +1,58 @@
+
+# Hook into unicorn, unicorn middleware, not rack middleware
+#
+# Since we need no knowledge about the request we can simply
+#  hook unicorn
+module Middleware::UnicornOobgc
+
+  MIN_REQUESTS_PER_OOBGC = 6
+  MAX_DELTAS = 20
+
+  def self.init
+    # hook up HttpServer intercept
+    ObjectSpace.each_object(Unicorn::HttpServer) do |s|
+      s.extend(self)
+    end
+  end
+
+  def process_client(client)
+    stat = GC.stat
+
+    @previous_deltas ||= []
+    @num_requests ||= 0
+    @num_requests += 1
+
+    # only track N deltas
+    if @previous_deltas.length > MAX_DELTAS
+      @previous_deltas.delete_at(0)
+    end
+
+    gc_count = stat[:count]
+    live_num = stat[:heap_live_num]
+
+    super(client) # Unicorn::HttpServer#process_client
+
+    # at this point client is serviced
+    stat = GC.stat
+    new_gc_count = stat[:count]
+    new_live_num = stat[:heap_live_num]
+
+    # no GC happened during the request
+    if new_gc_count == gc_count
+      @previous_deltas << (new_live_num - live_num)
+
+      if @gc_live_num && @num_requests > MIN_REQUESTS_PER_OOBGC
+        largest = @previous_deltas.max
+        if largest * (2 + Random.rand(2)) + new_live_num > @gc_live_num
+          GC.start
+          @num_requests = 0
+        end
+      end
+    else
+      puts "OobGC, GC live num adjusted, GC was not avoided: #{live_num}"
+      @gc_live_num = live_num
+    end
+
+  end
+
+end