class Traject::ThreadPool
An abstraction wrapping a Concurrent::ThreadPool in some configuration choices and other apparatus. Concurrent::ThreadPool is a Java ThreadPool
executor on jruby for performance, and is ruby-concurrent's own ruby implementation otherwise.
1) Initialize with chosen pool size – we create fixed size pools, where core and max sizes are the same.
2) If initialized with nil or 0 for threadcount, no thread pool will actually be created, and work sent to the Traject::ThreadPool
will just be executed in the caller thread. We call this a nil threadpool. One situation it can be useful is if you are running under MRI, where multi-core parallelism isn't available, so an actual threadpool may not be useful. (Although in some cases a thread pool, especially one with size 1, can be useful in MRI for I/O blocking operations)
3) Use the maybe_in_threadpool method to send blocks to thread pool for execution – if configurred with a nil threadcount, 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) We configure our underlying Concurrent::ThreadPool with a work queue that will buffer up to (pool_size
*3) tasks. If the queue is full, the underlying Concurrent::ThreadPool is set up to use the :caller_runs policy 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 :caller_runs policy 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, then queued or in-progress work may be abandoned when the program ends, which would be bad.
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)
Attributes
Public Class Methods
# File lib/traject/thread_pool.rb, line 59 def self.concurrency_disabled? ; @@disable_concurrency ; end
Calling Traject::ThreadPool.disable_concurrency!
permanently and irrevocably (for program execution) forces all ThreadPools to have a pool_size
of 0 – running all work inline – so should disable all use of threads in Traject
.
# File lib/traject/thread_pool.rb, line 58 def self.disable_concurrency! ; @@disable_concurrency = true ; end
First arg is pool size, 0 or nil and we'll be a null/no-op pool which executes work in caller thread.
# File lib/traject/thread_pool.rb, line 63 def initialize(pool_size) @thread_pool = nil # assume we don't have one @exceptions_caught_queue = [] # start off without exceptions if self.class.concurrency_disabled? pool_size = 0 end unless pool_size.nil? || pool_size == 0 @pool_size = pool_size.to_i @queue_capacity = pool_size * 3 @thread_pool = Concurrent::ThreadPoolExecutor.new( :min_threads => @pool_size, :max_threads => @pool_size, :max_queue => @queue_capacity, :fallback_policy => :caller_runs ) # A thread-safe queue to collect exceptions cross-threads. # We really only need to save the first exception, but a queue # is a convenient way to store a value concurrency-safely, and # might as well store all of them. @exceptions_caught_queue = Queue.new end end
Public Instance Methods
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.
# File lib/traject/thread_pool.rb, line 139 def collect_exception(e) @exceptions_caught_queue.push(e) end
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.
# File lib/traject/thread_pool.rb, line 119 def maybe_in_thread_pool(*args) if @thread_pool @thread_pool.post do begin yield(*args) rescue Exception => e collect_exception(e) end end else yield(*args) end end
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.
# File lib/traject/thread_pool.rb, line 150 def raise_collected_exception! unless @exceptions_caught_queue.empty? e = @exceptions_caught_queue.pop raise e end end
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
# File lib/traject/thread_pool.rb, line 162 def shutdown_and_wait start_t = Time.now if @thread_pool @thread_pool.shutdown @thread_pool.wait_for_termination end return (Time.now - start_t) end