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Class: Puma::ThreadPool

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Exceptions:
Inherits: Object
Defined in: lib/puma/thread_pool.rb

Overview

Internal Docs for A simple thread pool management object.

Each Puma “worker” has a thread pool to process requests.

First a connection to a client is made in Server. It is wrapped in a Client instance and then passed to the Reactor to ensure the whole request is buffered into memory. Once the request is ready, it is passed into a thread pool via the #<< operator where it is stored in a @todo array.

Each thread in the pool has an internal loop where it pulls a request from the @todo array and proceses it.

Constant Summary

Class Method Summary

Instance Attribute Summary

Instance Method Summary

Constructor Details

.new(min, max, *extra, &block) ⇒ ThreadPool

Maintain a minimum of min and maximum of max threads in the pool.

The block passed is the work that will be performed in each thread.

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 32

def initialize(min, max, *extra, &block)
  @not_empty = ConditionVariable.new
  @not_full = ConditionVariable.new
  @mutex = Mutex.new

  @todo = []

  @spawned = 0
  @waiting = 0

  @min = Integer(min)
  @max = Integer(max)
  @block = block
  @extra = extra

  @shutdown = false

  @trim_requested = 0
  @out_of_band_pending = false

  @workers = []

  @auto_trim = nil
  @reaper = nil

  @mutex.synchronize do
    @min.times do
      spawn_thread
      @not_full.wait(@mutex)
    end
  end

  @clean_thread_locals = false
  @force_shutdown = false
  @shutdown_mutex = Mutex.new
end

Class Method Details

.clean_thread_locals

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 73

def self.clean_thread_locals
  Thread.current.keys.each do |key| # rubocop: disable Performance/HashEachMethods
    Thread.current[key] = nil unless key == :__recursive_key__
  end
end

Instance Attribute Details

#busy_threads (readonly)

Version:

  • 5.0.0

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 92

def busy_threads
  with_mutex { @spawned - @waiting + @todo.size }
end

#clean_thread_locals (rw)

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 70

attr_accessor :clean_thread_locals

#out_of_band_hook (rw)

Version:

  • 5.0.0

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 71

attr_accessor :out_of_band_hook # @version 5.0.0

#pool_capacity (readonly)

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 86

def pool_capacity
  waiting + (@max - spawned)
end

#spawned (readonly)

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 69

attr_reader :spawned, :trim_requested, :waiting

#trim_requested (readonly)

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 69

attr_reader :spawned, :trim_requested, :waiting

#waiting (readonly)

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 69

attr_reader :spawned, :trim_requested, :waiting

Instance Method Details

#<<(work)

Add work to the todo list for a Thread to pickup and process.

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 183

def <<(work)
  with_mutex do
    if @shutdown
      raise "Unable to add work while shutting down"
    end

    @todo << work

    if @waiting < @todo.size and @spawned < @max
      spawn_thread
    end

    @not_empty.signal
  end
end

#auto_reap!(timeout = 5)

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 324

def auto_reap!(timeout=5)
  @reaper = Automaton.new(self, timeout, "threadpool reaper", :reap)
  @reaper.start!
end

#auto_trim!(timeout = 30)

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 319

def auto_trim!(timeout=30)
  @auto_trim = Automaton.new(self, timeout, "threadpool trimmer", :trim)
  @auto_trim.start!
end

#backlog

How many objects have yet to be processed by the pool?

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 81

def backlog
  with_mutex { @todo.size }
end

#reap

If there are dead threads in the pool make them go away while decreasing spawned counter so that new healthy threads could be created again.

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 277

def reap
  with_mutex do
    dead_workers = @workers.reject(&:alive?)

    dead_workers.each do |worker|
      worker.kill
      @spawned -= 1
    end

    @workers.delete_if do |w|
      dead_workers.include?(w)
    end
  end
end

#shutdown(timeout = -1))

Tell all threads in the pool to exit and wait for them to finish. Wait timeout seconds then raise ThreadPool::ForceShutdown in remaining threads. Next, wait an extra grace seconds then force-kill remaining threads. Finally, wait kill_grace seconds for remaining threads to exit.

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 347

def shutdown(timeout=-1)
  threads = with_mutex do
    @shutdown = true
    @trim_requested = @spawned
    @not_empty.broadcast
    @not_full.broadcast

    @auto_trim.stop if @auto_trim
    @reaper.stop if @reaper
    # dup workers so that we join them all safely
    @workers.dup
  end

  if timeout == -1
    # Wait for threads to finish without force shutdown.
    threads.each(&:join)
  else
    join = ->(inner_timeout) do
      start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
      threads.reject! do |t|
        elapsed = Process.clock_gettime(Process::CLOCK_MONOTONIC) - start
        t.join inner_timeout - elapsed
      end
    end

    # Wait timeout seconds for threads to finish.
    join.call(timeout)

    # If threads are still running, raise ForceShutdown and wait to finish.
    @shutdown_mutex.synchronize do
      @force_shutdown = true
      threads.each do |t|
        t.raise ForceShutdown if t[:with_force_shutdown]
      end
    end
    join.call(SHUTDOWN_GRACE_TIME)

    # If threads are _still_ running, forcefully kill them and wait to finish.
    threads.each(&:kill)
    join.call(1)
  end

  @spawned = 0
  @workers = []
end

#spawn_thread (private)

This method is for internal use only.

Must be called with @mutex held!

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 100

def spawn_thread
  @spawned += 1

  th = Thread.new(@spawned) do |spawned|
    Puma.set_thread_name 'threadpool %03i' % spawned
    todo  = @todo
    block = @block
    mutex = @mutex
    not_empty = @not_empty
    not_full = @not_full

    extra = @extra.map { |i| i.new }

    while true
      work = nil

      mutex.synchronize do
        while todo.empty?
          if @trim_requested > 0
            @trim_requested -= 1
            @spawned -= 1
            @workers.delete th
            Thread.exit
          end

          @waiting += 1
          if @out_of_band_pending && trigger_out_of_band_hook
            @out_of_band_pending = false
          end
          not_full.signal
          begin
            not_empty.wait mutex
          ensure
            @waiting -= 1
          end
        end

        work = todo.shift
      end

      if @clean_thread_locals
        ThreadPool.clean_thread_locals
      end

      begin
        @out_of_band_pending = true if block.call(work, *extra)
      rescue Exception => e
        STDERR.puts "Error reached top of thread-pool: #{e.message} (#{e.class})"
      end
    end
  end

  @workers << th

  th
end

#trigger_out_of_band_hook (private)

Version:

  • 5.0.0

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 160

def trigger_out_of_band_hook
  return false unless out_of_band_hook && out_of_band_hook.any?

  # we execute on idle hook when all threads are free
  return false unless @spawned == @waiting

  out_of_band_hook.each(&:call)
  true
rescue Exception => e
  STDERR.puts "Exception calling out_of_band_hook: #{e.message} (#{e.class})"
  true
end

#trim(force = false)

If there are any free threads in the pool, tell one to go ahead and exit. If force is true, then a trim request is requested even if all threads are being utilized.

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 265

def trim(force=false)
  with_mutex do
    free = @waiting - @todo.size
    if (force or free > 0) and @spawned - @trim_requested > @min
      @trim_requested += 1
      @not_empty.signal
    end
  end
end

#wait_for_less_busy_worker(delay_s)

Version:

  • 5.0.0

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 242

def wait_for_less_busy_worker(delay_s)
  # Ruby MRI does GVL, this can result
  # in processing contention when multiple threads
  # (requests) are running concurrently
  return unless Puma.mri?
  return unless delay_s > 0

  with_mutex do
    return if @shutdown

    # do not delay, if we are not busy
    return unless busy_threads > 0

    # this will be signaled once a request finishes,
    # which can happen earlier than delay
    @not_full.wait @mutex, delay_s
  end
end

#wait_until_not_full

This method is used by Server to let the server know when the thread pool can pull more requests from the socket and pass to the reactor.

The general idea is that the thread pool can only work on a fixed number of requests at the same time. If it is already processing that number of requests then it is at capacity. If another ::Puma process has spare capacity, then the request can be left on the socket so the other worker can pick it up and process it.

For example: if there are 5 threads, but only 4 working on requests, this method will not wait and the Server can pull a request right away.

If there are 5 threads and all 5 of them are busy, then it will pause here, and wait until the not_full condition variable is signaled, usually this indicates that a request has been processed.

It's important to note that even though the server might accept another request, it might not be added to the @todo array right away. For example if a slow client has only sent a header, but not a body then the @todo array would stay the same size as the reactor works to try to buffer the request. In that scenario the next call to this method would not block and another request would be added into the reactor by the server. This would continue until a fully bufferend request makes it through the reactor and can then be processed by the thread pool.

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 225

def wait_until_not_full
  with_mutex do
    while true
      return if @shutdown

      # If we can still spin up new threads and there
      # is work queued that cannot be handled by waiting
      # threads, then accept more work until we would
      # spin up the max number of threads.
      return if busy_threads < @max

      @not_full.wait @mutex
    end
  end
end

#with_force_shutdown

Allows ThreadPool::ForceShutdown to be raised within the provided block if the thread is forced to shutdown during execution.

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 331

def with_force_shutdown
  t = Thread.current
  @shutdown_mutex.synchronize do
    raise ForceShutdown if @force_shutdown
    t[:with_force_shutdown] = true
  end
  yield
ensure
  t[:with_force_shutdown] = false
end

#with_mutex(&block)

Version:

  • 5.0.0

[ GitHub ]

  
# File 'lib/puma/thread_pool.rb', line 176

def with_mutex(&block)
  @mutex.owned? ?
    yield :
    @mutex.synchronize(&block)
end