Class: Traject::ThreadPool

Inherits:
Object
  • Object
show all
Defined in:
lib/traject/thread_pool.rb

Overview

An abstraction wrapping a threadpool executor in some configuration choices and other apparatus.

1) Initialize with chosen pool size -- we create fixed size pools, where core and max sizes are the same.

2) If initialized with nil for threadcount, no thread pool will actually be created, and all threadpool-related methods become no-ops. We call this the nil/null threadpool. A non-nil threadpool requires jruby, but you can create a null Traject::ThreadPool.new(nil) under MRI without anything complaining.

3) Use the #maybe_in_threadpool method to send blocks to thread pool for execution -- if no threadpool configured your block will just be executed in calling thread. Be careful to not refer to any non-local variables in the block, unless the variable has an object you can use thread-safely!

4) Thread pools are java.util.concurrent.ThreadPoolExecutor, manually created with a work queue that will buffer up to (pool_size*3) tasks. If queue is full, the ThreadPoolExecutor is set up to use the ThreadPoolExecutor.CallerRunsPolicy, meaning the block will end up executing in caller's own thread. With the kind of work we're doing, where each unit of work is small and there are many of them-- the CallerRunsPolicy serves as an effective 'back pressure' mechanism to keep the work queue from getting too large and exhausting memory, when producers are faster than consumers.

5) Any exceptions raised by pool-executed work are captured accumulated in a thread-safe manner, and can be re-raised in the thread of your choice by calling #raise_collected_exception!

6) When you are done with the threadpool, you can and must call #shutdown_and_wait, which will wait for all current queued work to complete, then return. You can not give any more work to the pool after you do this. By default it'll wait pretty much forever, which should be fine. If you never call shutdown, the pool will keep running forever and not allow your program to exit!

7) We will keep track of total times a block is run in thread pool, and total elapsed (wall) time of running all blocks, so an average_execution_ms time can be given. #average_execution_ms may be inaccurate if called when threads are still executing, as it's not entirely thread safe (may get an off by one as to total iterations)

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(pool_size) ⇒ ThreadPool

First arg is pool size, 0 or nil and we'll be a null/no-op pool



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# File 'lib/traject/thread_pool.rb', line 49

def initialize(pool_size)
  unless pool_size.nil? || pool_size == 0
    require 'java' # trigger an exception now if we're not jruby

    @label = label

    @pool_size = pool_size.to_i # just for reflection, we don't really need it again
    @queue_capacity = pool_size * 3


    blockingQueue            =  java.util.concurrent.ArrayBlockingQueue.new(@queue_capacity)
    rejectedExecutionHandler =  java.util.concurrent.ThreadPoolExecutor::CallerRunsPolicy.new

    # keepalive times don't matter, we are setting core and max pool to
    # same thing, fixed size pool.
    @thread_pool =  java.util.concurrent.ThreadPoolExecutor.new(
      @pool_size, @pool_size, 0, java.util.concurrent.TimeUnit::MILLISECONDS,
      blockingQueue, rejectedExecutionHandler)

    # A thread-safe queue to collect exceptions cross-threads.
    # We make it small, we really only need to store the first
    # exception, we don't care too much about others. But we'll
    # keep the first 20, why not.
    @async_exception_queue   =  java.util.concurrent.ArrayBlockingQueue.new(20)
  end
end

Instance Attribute Details

#label ⇒ Object (readonly)

Returns the value of attribute label.



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# File 'lib/traject/thread_pool.rb', line 46

def label
  @label
end

#pool_size ⇒ Object (readonly)

Returns the value of attribute pool_size.



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# File 'lib/traject/thread_pool.rb', line 46

def pool_size
  @pool_size
end

#queue_capacity ⇒ Object (readonly)

Returns the value of attribute queue_capacity.



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# File 'lib/traject/thread_pool.rb', line 46

def queue_capacity
  @queue_capacity
end

Instance Method Details

#collect_exception(e) ⇒ Object

thread-safe way of storing an exception, to raise later in a different thread. We don't guarantee that we can store more than one at a time, only the first one recorded may be stored.



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# File 'lib/traject/thread_pool.rb', line 133

def collect_exception(e)
  # offer will silently do nothing if the queue is full, that's fine
  # with us.
  @async_exception_queue.offer(e)
end

#maybe_in_thread_pool(*args) ⇒ Object

Pass it a block, MAYBE gets executed in the bg in a thread pool. Maybe gets executed in the calling thread.

There are actually two 'maybes':

  • If Traject::ThreadPool was configured with null thread pool, then ALL work will be executed in calling thread.

  • If there is a thread pool, but it's work queue is full, then a job will be executed in calling thread (because we configured our java thread pool with a limited sized queue, and CallerRunsPolicy rejection strategy)

You can pass arbitrary arguments to the method, that will then be passed to your block -- similar to how ruby Thread.new works. This is convenient for creating variables unique to the block that won't be shared outside the thread:

thread_pool.maybe_in_thread_pool(x, y) do |x1, y1|
  100.times do
    something_with(x1)
  end
end
x = "someting else"
# If we hadn't passed args with block, and had just
# used x in the block, it'd be the SAME x as this one,
# and would be pointing to a different string now!

Note, that just makes block-local variables, it doesn't help you with whether a data structure itself is thread safe.



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# File 'lib/traject/thread_pool.rb', line 105

def maybe_in_thread_pool(*args)
  start_t = Time.now

  if @thread_pool
    @thread_pool.execute do
      begin
        yield(*args)
      rescue Exception => e
        collect_exception(e)
      end
    end
  else
    yield(*args)
  end

end

#queue ⇒ Object

Just for monitoring/debugging purposes, we'll return the work queue used by the threadpool. Don't recommend you do anything with it, as the original java.util.concurrent docs make the same recommendation.



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# File 'lib/traject/thread_pool.rb', line 125

def queue
  @thread_pool && @thread_pool.queue
end

#raise_collected_exception! ⇒ Object

If there's a stored collected exception, raise it again now. Call this to re-raise exceptions caught in other threads in the thread of your choice.

If you call this method on a ThreadPool initialized with nil as a non-functioning threadpool -- then this method is just a no-op.



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# File 'lib/traject/thread_pool.rb', line 146

def raise_collected_exception!
  if @async_exception_queue && e = @async_exception_queue.poll
    raise e
  end
end

#shutdown_and_wait ⇒ Object

shutdown threadpool, and wait for all work to complete. this one is also a no-op if you have a null ThreadPool that doesn't really have a threadpool at all.

returns elapsed time in seconds it took to shutdown



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# File 'lib/traject/thread_pool.rb', line 157

def shutdown_and_wait
  start_t = Time.now

  if @thread_pool
    @thread_pool.shutdown
    # We pretty much want to wait forever, although we need to give
    # a timeout. Okay, one day!
    @thread_pool.awaitTermination(1, java.util.concurrent.TimeUnit::DAYS)
  end

  return (Time.now - start_t)
end