ruby-concurrency/concurrent-ruby · error · ArgumentError

`min_threads` cannot be more than `max_threads`

Error message

`min_threads` cannot be more than `max_threads`

What it means

On JRuby, ThreadPoolExecutor raises ArgumentError when min_threads exceeds max_threads. The pool would otherwise be unable to satisfy both bounds, so construction fails fast during ns_initialize.

Source

Thrown at lib/concurrent-ruby/concurrent/executor/java_thread_pool_executor.rb:121

      def prune_pool
        deprecated "#prune_pool has no effect and will be removed in the next release."
      end

      private

      def ns_initialize(opts)
        min_length       = opts.fetch(:min_threads, DEFAULT_MIN_POOL_SIZE).to_i
        max_length       = opts.fetch(:max_threads, DEFAULT_MAX_POOL_SIZE).to_i
        idletime         = opts.fetch(:idletime, DEFAULT_THREAD_IDLETIMEOUT).to_i
        @max_queue       = opts.fetch(:max_queue, DEFAULT_MAX_QUEUE_SIZE).to_i
        @synchronous     = opts.fetch(:synchronous, DEFAULT_SYNCHRONOUS)
        @fallback_policy = opts.fetch(:fallback_policy, :abort)

        raise ArgumentError.new("`synchronous` cannot be set unless `max_queue` is 0") if @synchronous && @max_queue > 0
        raise ArgumentError.new("`max_threads` cannot be less than #{DEFAULT_MIN_POOL_SIZE}") if max_length < DEFAULT_MIN_POOL_SIZE
        raise ArgumentError.new("`max_threads` cannot be greater than #{DEFAULT_MAX_POOL_SIZE}") if max_length > DEFAULT_MAX_POOL_SIZE
        raise ArgumentError.new("`min_threads` cannot be less than #{DEFAULT_MIN_POOL_SIZE}") if min_length < DEFAULT_MIN_POOL_SIZE
        raise ArgumentError.new("`min_threads` cannot be more than `max_threads`") if min_length > max_length
        raise ArgumentError.new("#{fallback_policy} is not a valid fallback policy") unless FALLBACK_POLICY_CLASSES.include?(@fallback_policy)

        if @max_queue == 0
          if @synchronous
            queue = java.util.concurrent.SynchronousQueue.new
          else
            queue = java.util.concurrent.LinkedBlockingQueue.new
          end
        else
          queue = java.util.concurrent.LinkedBlockingQueue.new(@max_queue)
        end

        @executor = java.util.concurrent.ThreadPoolExecutor.new(
            min_length,
            max_length,
            idletime,
            java.util.concurrent.TimeUnit::SECONDS,
            queue,

View on GitHub (pinned to 0b88d5ff75)

Solutions

1. Set max_threads first and derive min_threads from it: min = [wanted_min, max].min
2. Validate the pair before construction and raise with both values in the message
3. Keep both values in one config block so they are reviewed together

Example fix

# before
Concurrent::ThreadPoolExecutor.new(min_threads: 10, max_threads: 5)
# after
max_threads = 5
min_threads = [10, max_threads].min
Concurrent::ThreadPoolExecutor.new(min_threads: min_threads, max_threads: max_threads)

max_threads = Integer(cfg.fetch(:max_threads, 2_147_483_647))
min_threads = [Integer(cfg.fetch(:min_threads, 0)), max_threads].min
raise ArgumentError, "min_threads #{min_threads} > max_threads #{max_threads}" if min_threads > max_threads
Concurrent::ThreadPoolExecutor.new(min_threads: min_threads, max_threads: max_threads)

def coherent_pool_bounds?(min, max)
  min.is_a?(Integer) && max.is_a?(Integer) && min >= 0 && max >= min && max <= 2_147_483_647
end

begin
  Concurrent::ThreadPoolExecutor.new(min_threads: mn, max_threads: mx)
rescue ArgumentError => e
  raise unless e.message.start_with?('`min_threads` cannot be more than')
  Concurrent::ThreadPoolExecutor.new(min_threads: [mn, mx].min, max_threads: mx)
end

When it happens

Trigger: min_threads: 10, max_threads: 5 on JRuby; independently configured values (MIN defaulting high while MAX is lowered); YAML overrides applied in the wrong precedence order.

Common situations: Two ENV vars or YAML keys maintained by different teams drifting apart; lowering max_threads in an environment-specific override while min_threads comes from a shared default; renames that swap the two keys.

Related errors

• `synchronous` cannot be set unless `max_queue` is 0
• `max_threads` cannot be less than #{DEFAULT_MIN_POOL_SIZE}
• `max_threads` cannot be greater than #{DEFAULT_MAX_POOL_SIZE
• `min_threads` cannot be less than #{DEFAULT_MIN_POOL_SIZE}
• #{fallback_policy} is not a valid fallback policy

AI-assisted analysis of ruby-concurrency/concurrent-ruby@0b88d5ff75 (2026-08-21). Data as JSON: /api/errors/9b7f33231df0c319. Report an issue: GitHub.