test.rb

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