Allow use of Puma's automatic worker count configuration

Gemfile

@@ -4,6 +4,7 @@ gem 'addressable'
 gem 'allowy', '>= 2.1.0'
 gem 'clockwork', require: false
 gem 'cloudfront-signer'
+gem 'concurrent-ruby'
 gem 'digest-xxhash'
 gem 'eventmachine', '~> 1.2.7'
 gem 'fluent-logger'

Gemfile.lock

@@ -595,6 +595,7 @@ DEPENDENCIES
   clockwork
   cloudfront-signer
   codeclimate-test-reporter (>= 1.0.8)
+  concurrent-ruby
   debug (~> 1.10)
   digest-xxhash
   eventmachine (~> 1.2.7)

lib/cloud_controller/config_schemas/base/api_schema.rb

@@ -267,6 +267,7 @@ class ApiSchema < VCAP::Config
 
             webserver: String, # thin or puma
             optional(:puma) => {
+              automatic_worker_count: bool,
               workers: Integer,
               max_threads: Integer,
               optional(:max_db_connections_per_process) => Integer

lib/cloud_controller/runners/puma_runner.rb

@@ -8,6 +8,7 @@ class PumaRunner
     def initialize(config, app, logger, periodic_updater, request_logs)
       @logger = logger
 
+      ENV['WEB_CONCURRENCY'] = 'auto' if config.get(:puma, :automatic_worker_count)
       puma_config = Puma::Configuration.new do |conf|
         if config.get(:nginx, :use_nginx)
           if config.get(:nginx, :instance_socket).nil? || config.get(:nginx, :instance_socket).empty?
@@ -19,7 +20,7 @@ def initialize(config, app, logger, periodic_updater, request_logs)
           conf.bind "tcp://0.0.0.0:#{config.get(:external_port)}"
         end
 
-        conf.workers(config.get(:puma, :workers) || 1)
+        conf.workers(config.get(:puma, :workers) || 1) unless config.get(:puma, :automatic_worker_count)
         num_threads = config.get(:puma, :max_threads) || 1
         conf.threads(num_threads, num_threads)
 

spec/unit/lib/cloud_controller/runners/puma_runner_spec.rb

@@ -15,7 +15,7 @@ module VCAP::CloudController
     let(:dependency_locator) { instance_spy(CloudController::DependencyLocator) }
     let(:prometheus_updater) { spy(VCAP::CloudController::Metrics::PrometheusUpdater) }
 
-    subject do
+    let(:test_config) do
       TestConfig.override(
         external_port: port,
         nginx: {
@@ -27,7 +27,10 @@ module VCAP::CloudController
           max_threads: max_threads
         }
       )
-      PumaRunner.new(TestConfig.config_instance, app, logger, periodic_updater, request_logs)
+    end
+
+    subject do
+      PumaRunner.new(test_config, app, logger, periodic_updater, request_logs)
     end
 
     before do
@@ -93,6 +96,52 @@ module VCAP::CloudController
         end
       end
 
+      context 'when setting "automatic_worker_count" to false' do
+        let(:test_config) do
+          TestConfig.override(
+            puma: {
+              workers: 1,
+              automatic_worker_count: false
+            }
+          )
+        end
+
+        before do
+          allow(::Concurrent).to receive(:available_processor_count).and_return 8
+        end
+
+        it 'configures number of workers to the detected number of cores' do
+          subject
+
+          expect(puma_launcher.config.final_options[:workers]).to eq(1)
+          expect(puma_launcher.config.final_options[:min_threads]).to eq(1)
+          expect(puma_launcher.config.final_options[:max_threads]).to eq(1)
+        end
+      end
+
+      context 'when setting "automatic_worker_count" to true' do
+        let(:test_config) do
+          TestConfig.override(
+            puma: {
+              workers: 1,
+              automatic_worker_count: true
+            }
+          )
+        end
+
+        before do
+          allow(::Concurrent).to receive(:available_processor_count).and_return 8
+        end
+
+        it 'configures number of workers the specified number of workers' do
+          subject
+
+          expect(puma_launcher.config.final_options[:workers]).to eq(8)
+          expect(puma_launcher.config.final_options[:min_threads]).to eq(1)
+          expect(puma_launcher.config.final_options[:max_threads]).to eq(1)
+        end
+      end
+
       it 'configures the app as middleware' do
         subject