CachedThreadPool does not spin down idle threads

[SamSaffron]

In the Ruby implementation:

require 'concurrent'

puts "Thread Count: #{Thread.list.count}"

pool = Concurrent::CachedThreadPool.new( min_threads: 0, max_threads: 5, idletime: 1 )

puts "Thread Count: #{Thread.list.count}"
# we have 1 thread for now

5.times do
  pool.post do
    puts "Hello from thread #{Thread.current.object_id}"
  end
end
# we now have 5

sleep 2
puts "Thread Count: #{Thread.list.count}"
# we still have 5

pool.post do
  puts "Hello from thread #{Thread.current.object_id}"
end

sleep 1

puts "Thread Count: #{Thread.list.count}"
# we just issued a prune as a side effect so we have 2

pool.prune_pool
sleep 2
# we need to sleep cause prune_pool is async

puts "Thread Count: #{Thread.list.count}"
# we now have 1 thread

CachedThreadPool only spins down threads when you queue work, so if you are done queuing work you need to remember to wait for it to be done and then issue a #prune_pool call

This is technically very tricky cause you need to carry an extra thread for that. The Ruby design should change so threads just wait idle_time for new work and spin down if no new work arrives.

[bensheldon]

What's the cost/downside you're seeing with a sleeping thread on Queue#pop?

To prune precisely I think we have a couple of options:

• another thread to sleep and prune. Downside as you note is another thread.
• replace the Queue object with a queue-with-wait implementation (I don't think Concurrent Ruby has one, though TimerSet is close it) so the thread could prune itself.

[SamSaffron]

@bensheldon mainly complexity, it is complexity we do not want to carry.

I wrote a queue with a conditional var here:

discourse/discourse#30364

I am more than happy for this to be lifted into concurrent ruby, it has a few tiny discoursisms (multisite support and logging patterns) but is safe.

For us this is perfect, I have not benched it against the current implementation but for our usage we always tend to make DB calls so we are not optimising for microsecond execution times shortest would be usually 10s of milliseconds.

[eregon]

I wrote a queue with a conditional var here:

FWIW I also recall writing one in https://github.com/ruby/timeout/blob/607d8c6fbe4d86db1cf22846e89198b47cec7161/lib/timeout.rb#L105-L109
And since Ruby 3.2 there is Queue#pop(timeout:).

[SamSaffron]

@eregon thanks for the tip, I ended up following up with a hybrid approach

https://github.com/discourse/discourse/pull/30392/files

There is so much detail in such a small amount of code. being able to efficiently scale up the queue is very tough, coordinating work between all the critical sections is tough.

I am not even sure if this is my last PR refining this ...

Would love it if you can have a look and let me know if this is something we should port to concurrent, having a building block like this is super important, I want it to work for everyone

[eregon]

I think having the improvement to automatically spin down idle threads in CachedThreadPool would make a lot of sense.
The only thing is I don't have much time to review, but other maintainers might have more.

It seems the Java version does that:
https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Executors.html#newCachedThreadPool--

Threads that have not been used for sixty seconds are terminated and removed from the cache.

So it would be good for consistency too.

[joshuay03]

Hey folks, I was pointed this way after opening a PR to fix a similar issue with ThreadPoolExecutor. I ended up adding another thread, however I'm open to other ideas. This seems worth exploring 🤔:

replace the Queue object with a queue-with-wait implementation (I don't think Concurrent Ruby has one, though TimerSet is close it) so the thread could prune itself.

Context:

• Rails PR: Use a fixed worker pool size for Action Cable server rails/rails#53211
• CR issue: Unexpected pruning behaviour with consecutive task batches #1066
• CR PR: Less aggressive async pruning for RubyThreadPoolExecutor #1079

[bensheldon]

TIL about Queue#pop(timeout:) after Ruby 3.2. That is soo nice!

...but also presents the challenge that it becomes simple (imo) to implement spinning down idle threads in Ruby 3.2+... and somewhat complex to do so in older versions of Ruby (assuming that a timeoutable-queue-pop is the strategy).

I sorta see two paths, and I'd love feedback about which to pick:

• Have idle thread spin-down be a Ruby 3.2+ only feature
• Have a Concurrent::QueueWithPopTimeout that implements the feature on older Ruby and inherits from Queue for Ruby 3.2+

[SamSaffron]

There is a 100% working implementation using ConditionVariable.new right here:

https://github.com/discourse/discourse/blob/main/lib/scheduler/thread_pool.rb

No background thread needed, Queue timeout is awesome, but this achieves the exact same goal in a backwards compatible way.

The challenge here imo is not the implementation, it is more the architecture of Concurrent rb. Given its Java roots there are lots of abstractions at play and conditional Java code baked in. It makes it very complicated to port in a solution like this despite it being pretty much ready.