Better pruning for RubyThreadPoolExecutor

lib/concurrent-ruby/concurrent/collection/ruby_timeout_queue.rb

@@ -0,0 +1,44 @@
+module Concurrent
+  module Collection
+    # @!visibility private
+    # @!macro ruby_timeout_queue
+    class RubyTimeoutQueue < ::Queue
+      def initialize(*args)
+        if RUBY_VERSION >= '3.2'
+          raise 'RubyTimeoutQueue is not needed on Ruby 3.2 or later, use ::Queue instead'
+        end
+
+        super(*args)
+
+        @mutex = Mutex.new
+        @cond_var = ConditionVariable.new
+      end
+
+      def push(obj)
+        super(obj).tap { @mutex.synchronize { @cond_var.signal } }
+      end
+      alias_method :enq, :push
+      alias_method :<<, :push
+
+      def pop(non_block = false, timeout: nil)
+        if non_block && timeout
+          raise ArgumentError, "can't set a timeout if non_block is enabled"
+        end
+
+        if non_block
+          super(true)
+        elsif timeout && empty?
+          if @mutex.synchronize { @cond_var.wait(@mutex, timeout) }
+            super(false)
+          else
+            nil
+          end
+        else
+          super(false)
+        end
+      end
+      alias_method :deq, :pop
+      alias_method :shift, :pop
+    end
+  end
+end

lib/concurrent-ruby/concurrent/collection/timeout_queue.rb

@@ -0,0 +1,18 @@
+module Concurrent
+  module Collection
+    # @!visibility private
+    # @!macro internal_implementation_note
+    TimeoutQueueImplementation = if RUBY_VERSION >= '3.2'
+                                   ::Queue
+                                 else
+                                   require 'concurrent/collection/ruby_timeout_queue'
+                                   RubyTimeoutQueue
+                                 end
+    private_constant :TimeoutQueueImplementation
+
+    # @!visibility private
+    # @!macro timeout_queue
+    class TimeoutQueue < TimeoutQueueImplementation
+    end
+  end
+end

lib/concurrent-ruby/concurrent/executor/fixed_thread_pool.rb

@@ -75,17 +75,6 @@ module Concurrent
   #   @return [Integer] Number of tasks that may be enqueued before reaching `max_queue` and rejecting
   #     new tasks. A value of -1 indicates that the queue may grow without bound.
 
-  # @!macro thread_pool_executor_method_prune_pool
-  #   Prune the thread pool of unneeded threads
-  #
-  #   What is being pruned is controlled by the min_threads and idletime
-  #   parameters passed at pool creation time
-  #
-  #   This is a no-op on some pool implementation (e.g. the Java one).  The Ruby
-  #   pool will auto-prune each time a new job is posted. You will need to call
-  #   this method explicitly in case your application post jobs in bursts (a
-  #   lot of jobs and then nothing for long periods)
-
   # @!macro thread_pool_executor_public_api
   #
   #   @!macro abstract_executor_service_public_api
@@ -122,9 +111,6 @@ module Concurrent
   #
   #   @!method can_overflow?
   #     @!macro executor_service_method_can_overflow_question
-  #
-  #   @!method prune_pool
-  #     @!macro thread_pool_executor_method_prune_pool
 
 
 

lib/concurrent-ruby/concurrent/executor/java_thread_pool_executor.rb

@@ -98,10 +98,6 @@ def running?
         super && !@executor.isTerminating
       end
 
-      # @!macro thread_pool_executor_method_prune_pool
-      def prune_pool
-      end
-
       private
 
       def ns_initialize(opts)

lib/concurrent-ruby/concurrent/executor/ruby_thread_pool_executor.rb

@@ -3,6 +3,7 @@
 require 'concurrent/concern/logging'
 require 'concurrent/executor/ruby_executor_service'
 require 'concurrent/utility/monotonic_time'
+require 'concurrent/collection/timeout_queue'
 
 module Concurrent
 
@@ -104,6 +105,11 @@ def ready_worker(worker, last_message)
       synchronize { ns_ready_worker worker, last_message }
     end
 
+    # @!visibility private
+    def remove_ready_worker(worker)
+      synchronize { ns_remove_ready_worker worker }
+    end
+
     # @!visibility private
     def worker_died(worker)
       synchronize { ns_worker_died worker }
@@ -114,9 +120,9 @@ def worker_task_completed
       synchronize { @completed_task_count += 1 }
     end
 
-    # @!macro thread_pool_executor_method_prune_pool
-    def prune_pool
-      synchronize { ns_prune_pool }
+    # @!visibility private
+    def prunable_capacity
+      synchronize { ns_prunable_capacity }
     end
 
     private
@@ -146,9 +152,6 @@ def ns_initialize(opts)
       @largest_length       = 0
       @workers_counter      = 0
       @ruby_pid             = $$ # detects if Ruby has forked
-
-      @gc_interval  = opts.fetch(:gc_interval, @idletime / 2.0).to_i # undocumented
-      @next_gc_time = Concurrent.monotonic_time + @gc_interval
     end
 
     # @!visibility private
@@ -162,12 +165,10 @@ def ns_execute(*args, &task)
 
       if ns_assign_worker(*args, &task) || ns_enqueue(*args, &task)
         @scheduled_task_count += 1
+        nil
       else
-        return fallback_action(*args, &task)
+        fallback_action(*args, &task)
       end
-
-      ns_prune_pool if @next_gc_time < Concurrent.monotonic_time
-      nil
     end
 
     # @!visibility private
@@ -218,7 +219,7 @@ def ns_assign_worker(*args, &task)
     # @!visibility private
     def ns_enqueue(*args, &task)
       return false if @synchronous
-      
+
       if !ns_limited_queue? || @queue.size < @max_queue
         @queue << [task, args]
         true
@@ -265,7 +266,7 @@ def ns_ready_worker(worker, last_message, success = true)
       end
     end
 
-    # removes a worker which is not in not tracked in @ready
+    # removes a worker which is not tracked in @ready
     #
     # @!visibility private
     def ns_remove_busy_worker(worker)
@@ -274,23 +275,13 @@ def ns_remove_busy_worker(worker)
       true
     end
 
-    # try oldest worker if it is idle for enough time, it's returned back at the start
-    #
-    # @!visibility private
-    def ns_prune_pool
-      now = Concurrent.monotonic_time
-      stopped_workers = 0
-      while !@ready.empty? && (@pool.size - stopped_workers > @min_length)
-        worker, last_message = @ready.first
-        if now - last_message > self.idletime
-          stopped_workers += 1
-          @ready.shift
-          worker << :stop
-        else break
-        end
-      end
+    def ns_remove_ready_worker(worker)
+      @ready.delete_if { |rw, _| rw == worker }
+      true
+    end
 
-      @next_gc_time = Concurrent.monotonic_time + @gc_interval
+    def ns_prunable_capacity
+      [@pool.size - @min_length, @ready.size].min
     end
 
     def ns_reset_if_forked
@@ -312,7 +303,7 @@ class Worker
 
       def initialize(pool, id)
         # instance variables accessed only under pool's lock so no need to sync here again
-        @queue  = Queue.new
+        @queue  = Collection::TimeoutQueue.new
         @pool   = pool
         @thread = create_worker @queue, pool, pool.idletime
 
@@ -338,17 +329,29 @@ def kill
       def create_worker(queue, pool, idletime)
         Thread.new(queue, pool, idletime) do |my_queue, my_pool, my_idletime|
           catch(:stop) do
-            loop do
+            not_prunable = false
 
-              case message = my_queue.pop
+            loop do
+              timeout = not_prunable ? nil : my_idletime
+              case message = my_queue.pop(timeout: timeout)
+              when nil
+                unless my_pool.prunable_capacity.positive?
+                  not_prunable = true
+                  next
+                end
+
+                my_pool.remove_ready_worker(self)
+                my_pool.remove_busy_worker(self)
+                throw :stop
               when :stop
+                my_pool.remove_ready_worker(self)
                 my_pool.remove_busy_worker(self)
                 throw :stop
-
               else
                 task, args = message
                 run_task my_pool, task, args
                 my_pool.ready_worker(self, Concurrent.monotonic_time)
+                not_prunable = false
               end
             end
           end

spec/concurrent/executor/cached_thread_pool_spec.rb

@@ -152,15 +152,12 @@ module Concurrent
 
         context 'garbage collection' do
 
-          subject { described_class.new(idletime: 0.1, max_threads: 2, gc_interval: 0) }
+          subject { described_class.new(idletime: 0.1, max_threads: 2) }
 
           it 'removes from pool any thread that has been idle too long' do
             latch = Concurrent::CountDownLatch.new(4)
             4.times { subject.post { sleep 0.1; latch.count_down } }
             expect(latch.wait(1)).to be true
-            sleep 0.2
-            subject.post {}
-            sleep 0.2
             expect(subject.length).to be < 4
           end
 
@@ -197,25 +194,27 @@ module Concurrent
             expect(subject.length).to be >= 5
             3.times { subject << proc { sleep(1) } }
             sleep(0.1)
-            expect(subject.length).to be >= 5
+            expect(subject.length).to be >= 3
           end
         end
       end
 
       context 'stress' do
         configurations = [
-            { min_threads:    2,
-              max_threads:    ThreadPoolExecutor::DEFAULT_MAX_POOL_SIZE,
-              idletime:       0.1, # 1 minute
-              max_queue: 0, # unlimited
+            {
+              min_threads:     2,
+              max_threads:     ThreadPoolExecutor::DEFAULT_MAX_POOL_SIZE,
+              idletime:        60, # 1 minute
+              max_queue:       0, # unlimited
               fallback_policy: :caller_runs, # shouldn't matter -- 0 max queue
-              gc_interval: 0.1 },
-            { min_threads:    2,
-              max_threads:    4,
-              idletime:       0.1, # 1 minute
-              max_queue: 0, # unlimited
+            },
+            {
+              min_threads:     2,
+              max_threads:     4,
+              idletime:        60, # 1 minute
+              max_queue:       0, # unlimited
               fallback_policy: :caller_runs, # shouldn't matter -- 0 max queue
-              gc_interval: 0.1 }
+            }
         ]
 
         configurations.each do |config|

spec/concurrent/executor/java_thread_pool_executor_spec.rb

@@ -26,13 +26,6 @@ module Concurrent
 
       it_should_behave_like :thread_pool_executor
 
-      context :prune do
-        it "is a no-op, pruning is handled by the JVM" do
-          executor = JavaThreadPoolExecutor.new
-          executor.prune_pool
-        end
-      end
-
       context '#overload_policy' do
 
         specify ':abort maps to AbortPolicy' do