Asynchronous pruning for RubyThreadPoolExecutor

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

@@ -0,0 +1,55 @@
+module Concurrent
+  module Collection
+    # @!visibility private
+    # @!macro ruby_timeout_queue
+    class RubyTimeoutQueue < ::Queue
+      def initialize(*args)
+        if RUBY_VERSION >= '3.2'
+          raise "#{self.class.name} 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)
+        @mutex.synchronize do
+          super(obj)
+          @cond_var.signal
+        end
+      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
+          @mutex.synchronize do
+            deadline = Concurrent.monotonic_time + timeout
+            while (now = Concurrent.monotonic_time) < deadline && empty?
+              @cond_var.wait(@mutex, deadline - now)
+            end
+            begin
+              return super(true)
+            rescue ThreadError
+              # still empty
+              nil
+            end
+          end
+        else
+          super(false)
+        end
+      end
+      alias_method :deq, :pop
+      alias_method :shift, :pop
+    end
+    private_constant :RubyTimeoutQueue
+  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

@@ -81,10 +81,8 @@ module Concurrent
   #   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)
+  #   This is a no-op on all pool implementations as they prune themselves
+  #   automatically, and has been deprecated.
 
   # @!macro thread_pool_executor_public_api
   #

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

@@ -8,6 +8,7 @@ module Concurrent
     # @!macro thread_pool_options
     # @!visibility private
     class JavaThreadPoolExecutor < JavaExecutorService
+      include Concern::Deprecation
 
       # @!macro thread_pool_executor_constant_default_max_pool_size
       DEFAULT_MAX_POOL_SIZE = java.lang.Integer::MAX_VALUE # 2147483647
@@ -100,6 +101,7 @@ def running?
 
       # @!macro thread_pool_executor_method_prune_pool
       def prune_pool
+        deprecated "#prune_pool has no effect and will be removed in the next release."
       end
 
       private

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

@@ -3,13 +3,15 @@
 require 'concurrent/concern/logging'
 require 'concurrent/executor/ruby_executor_service'
 require 'concurrent/utility/monotonic_time'
+require 'concurrent/collection/timeout_queue'
 
 module Concurrent
 
   # @!macro thread_pool_executor
   # @!macro thread_pool_options
   # @!visibility private
   class RubyThreadPoolExecutor < RubyExecutorService
+    include Concern::Deprecation
 
     # @!macro thread_pool_executor_constant_default_max_pool_size
     DEFAULT_MAX_POOL_SIZE      = 2_147_483_647 # java.lang.Integer::MAX_VALUE
@@ -94,9 +96,28 @@ def remaining_capacity
       end
     end
 
+    # removes the worker if it can be pruned
+    #
+    # @return [true, false] if the worker was pruned
+    #
     # @!visibility private
-    def remove_busy_worker(worker)
-      synchronize { ns_remove_busy_worker worker }
+    def prune_worker(worker)
+      synchronize do
+        if ns_prunable_capacity > 0
+          remove_worker worker
+          true
+        else
+          false
+        end
+      end
+    end
+
+    # @!visibility private
+    def remove_worker(worker)
+      synchronize do
+        ns_remove_ready_worker worker
+        ns_remove_busy_worker worker
+      end
     end
 
     # @!visibility private
@@ -116,7 +137,7 @@ def worker_task_completed
 
     # @!macro thread_pool_executor_method_prune_pool
     def prune_pool
-      synchronize { ns_prune_pool }
+      deprecated "#prune_pool has no effect and will be removed in next the release, see https://github.com/ruby-concurrency/concurrent-ruby/pull/1082."
     end
 
     private
@@ -146,9 +167,6 @@ 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 +180,10 @@ 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 +234,7 @@ 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 +281,7 @@ 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)
@@ -274,25 +290,27 @@ 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 [email protected]? && (@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
+    def ns_remove_ready_worker(worker)
+      if index = @ready.index { |rw, _| rw == worker }
+        @ready.delete_at(index)
       end
+      true
+    end
 
-      @next_gc_time = Concurrent.monotonic_time + @gc_interval
+    # @return [Integer] number of excess idle workers which can be removed without
+    #                   going below min_length, or all workers if not running
+    #
+    # @!visibility private
+    def ns_prunable_capacity
+      if running?
+        [@pool.size - @min_length, @ready.size].min
+      else
+        @pool.size
+      end
     end
 
+    # @!visibility private
     def ns_reset_if_forked
       if $$ != @ruby_pid
         @queue.clear
@@ -312,7 +330,7 @@ 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 +356,22 @@ 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
+            prunable = true
 
-              case message = my_queue.pop
+            loop do
+              timeout = prunable && my_pool.running? ? my_idletime : nil
+              case message = my_queue.pop(timeout: timeout)
+              when nil
+                throw :stop if my_pool.prune_worker(self)
+                prunable = false
               when :stop
-                my_pool.remove_busy_worker(self)
+                my_pool.remove_worker(self)
                 throw :stop
-
               else
                 task, args = message
                 run_task my_pool, task, args
                 my_pool.ready_worker(self, Concurrent.monotonic_time)
+                prunable = true
               end
             end
           end

spec/concurrent/executor/cached_thread_pool_spec.rb

@@ -152,15 +152,13 @@ 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 } }
+            sleep 0.4
             expect(latch.wait(1)).to be true
-            sleep 0.2
-            subject.post {}
-            sleep 0.2
             expect(subject.length).to be < 4
           end
 
@@ -197,25 +195,27 @@ 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