Promote worker process to optimize copy-on-write performance

Add a hook (SIGURG) to promote an existing worker process to become
a new master, and spawn a new set of forked worker processes.
Because the promoted worker has been serving live requests, forking from
this process instead of the initial preloaded app can potentially
improve overall copy-on-write performance and reduce memory usage.

History.md

@@ -5,6 +5,7 @@
   * Configuration: `environment` is read from `RAILS_ENV`, if `RACK_ENV` can't be found (#2022)
   * `Puma.stats` now returns a Hash instead of a JSON string (#2086)
   * `GC.compact` is called before fork if available (#2093)
+  * Add command to `promote` worker process to optimize copy-on-write performance (#2099)
 
 * Bugfixes
   * Your bugfix goes here (#Github Number)

lib/puma/app/status.rb

@@ -63,6 +63,11 @@ def call(env)
           end
 
           rack_response(200, backtraces.to_json)
+
+        when /\/promote$/
+          Process.kill "SIGURG", $$
+          rack_response(200, OK_STATUS)
+
         else
           rack_response 404, "Unsupported action", 'text/plain'
         end

lib/puma/cluster.rb

@@ -127,6 +127,12 @@ def hup
         Process.kill "HUP", @pid
       rescue Errno::ESRCH
       end
+
+      def promote
+        Process.kill "URG", @pid
+        Process.detach @pid
+      rescue Errno::ESRCH
+      end
     end
 
     def spawn_workers
@@ -249,7 +255,7 @@ def worker(index, master)
       @master_read.close
       @suicide_pipe.close
 
-      Thread.new do
+      parent_check = Thread.new do
         Puma.set_thread_name "worker check pipe"
         IO.select [@check_pipe]
         log "! Detected parent died, dying"
@@ -277,6 +283,13 @@ def worker(index, master)
         server.stop
       end
 
+      # Promote worker
+      Signal.trap "SIGURG" do
+        @promoted = true
+        parent_check.kill
+        server.begin_restart(true)
+      end
+
       begin
         @worker_write << "b#{Process.pid}\n"
       rescue SystemCallError, IOError
@@ -313,6 +326,7 @@ def worker(index, master)
     ensure
       @worker_write << "t#{Process.pid}\n" rescue nil
       @worker_write.close
+      @launcher.run if @promoted
     end
 
     def restart
@@ -431,7 +445,7 @@ def run
 
       if preload?
         log "* Preloading application"
-        load_and_bind
+        load_and_bind unless @promoted
 
         after = Thread.list
 
@@ -454,7 +468,7 @@ def run
           exit 1
         end
 
-        @launcher.binder.parse @options[:binds], self
+        @launcher.binder.parse @options[:binds], self unless @promoted
       end
 
       read, @wakeup = Puma::Util.pipe
@@ -488,12 +502,18 @@ def run
       @launcher.config.run_hooks :before_fork, nil
       GC.compact if GC.respond_to?(:compact)
 
+      @promoted = false
       spawn_workers
 
       Signal.trap "SIGINT" do
         stop
       end
 
+      Signal.trap "SIGURG" do
+        @promoted = @workers.shift
+        stop
+      end
+
       @launcher.events.fire_on_booted!
 
       begin
@@ -545,6 +565,7 @@ def run
           end
         end
 
+        promote if @promoted
         stop_workers unless @status == :halt
       ensure
         @check_pipe.close
@@ -556,6 +577,12 @@ def run
 
     private
 
+    def promote
+      @control.stop(true) if @control
+      close_control_listeners
+      @launcher.promote
+      @promoted.promote
+    end
     # loops thru @workers, removing workers that exited, and calling
     # `#term` if needed
     def wait_workers

lib/puma/control_cli.rb

@@ -11,7 +11,7 @@
 module Puma
   class ControlCLI
 
-    COMMANDS = %w{halt restart phased-restart start stats status stop reload-worker-directory gc gc-stats thread-backtraces}
+    COMMANDS = %w{halt restart phased-restart start stats status stop reload-worker-directory gc gc-stats thread-backtraces promote}
     PRINTABLE_COMMANDS = %w{gc-stats stats thread-backtraces}
 
     def initialize(argv, stdout=STDOUT, stderr=STDERR)
@@ -211,6 +211,9 @@ def send_signal
         when "stop"
           Process.kill "SIGTERM", @pid
 
+        when "promote"
+          Process.kill "SIGURG", @pid
+
         when "stats"
           puts "Stats not available via pid only"
           return

lib/puma/launcher.rb

@@ -141,6 +141,10 @@ def restart
       @runner.restart
     end
 
+    def promote
+      @status = :promote
+    end
+
     # Begin a phased restart if supported
     def phased_restart
       unless @runner.respond_to?(:phased_restart) and @runner.phased_restart
@@ -184,7 +188,7 @@ def run
       when :exit
         # nothing
       end
-      close_binder_listeners unless @status == :restart
+      close_binder_listeners unless @status == :restart || @status == :promote
     end
 
     # Return which tcp port the launcher is using, if it's using TCP

lib/puma/server.rb

@@ -1006,8 +1006,9 @@ def halt(sync=false)
       @thread.join if @thread && sync
     end
 
-    def begin_restart
+    def begin_restart(sync=false)
       notify_safely(RESTART_COMMAND)
+      @thread.join if @thread && sync
     end
 
     def fast_write(io, str)

test/helpers/integration.rb

@@ -28,13 +28,14 @@ def teardown
       io = nil
     end
     refute File.exist?(@bind_path), "Bind path must be removed after stop"
-    File.unlink(@bind_path) rescue nil
 
     # wait until the end for OS buffering?
     if defined?(@server) && @server
       @server.close unless @server.closed?
       @server = nil
     end
+  ensure
+    File.unlink(@bind_path) rescue nil
   end
 
   private

test/test_integration_pumactl.rb

@@ -105,6 +105,24 @@ def test_kill_unknown
     assert_equal(1, e.status)
   end
 
+  def test_promote
+    skip NO_FORK_MSG unless HAS_FORK
+    skip UNIX_SKT_MSG unless UNIX_SKT_EXIST
+
+    cli_server "-w #{WORKERS} test/rackup/sleep.ru --control-url unix://#{@control_path} --control-token #{TOKEN} -S #{@state_path}", unix: true
+    cli_pumactl "promote", unix: true
+
+    _, status = Process.wait2(@pid)
+    assert_equal 0, status
+
+    new_pid = File.read(@state_path).match(/pid: (\d+)/m)[1].to_i
+    refute_equal @pid, new_pid
+
+    cli_pumactl "stop", unix: true
+    sleep 0.1 while File.exist?(@bind_path)
+    @server = nil
+  end
+
   private
 
   def cli_pumactl(argv, unix: false)