[ruby-core:124907] [Ruby Feature#21930] Add Ractor#empty? method to check for pending messages without blocking

3 Mar 2026

      Issue #21930 has been updated by synacker (Mikhail Milovidov).

nobu (Nobuyoshi Nakada) wrote in #note-3:
...
As for your example, why does `Worker` ractor handle both of `main_task` and dispatch alone, instead of launching each ractors?
I compared two strategies for handling concurrent tasks:
1. run1: Ractors + fibers
2. run2: only ractors

Here’s the benchmark code that executed 10000 tasks using both strategies and measured real execution time::
```ruby
require 'async'
require 'benchmark'

class TimeCommand
    attr_reader :id

    def initialize(id)
        @id = id
    end

    def task
        1.upto(3) do |i|      
            sleep(1)
            "[cmd #{@id}] step #{i} @ #{Time.now}"
        end
    end
end

class StopCommand
end

class Worker
    attr_reader :iterations

    def initialize(iterations: 1000)
        @iterations = iterations
    end

    def run1
        ractor = Ractor.new do
            loop do
                run = true
                Sync do |task|
                    in_queue = Async::Queue.new
                    queue_task = task.async do |subtask|
                        while command = in_queue.dequeue
                            subtask.async do |child_task|
                                command.task
                            end
                            if command.is_a?(StopCommand)
                                break
                            end
                        end
                    end

                    task.async(transient: true) do |main_task|
                        loop do
                            commands = []
                            if queue_task.children? || !in_queue.empty?
                                main_task.yield
                                commands.append Ractor.receive while !Ractor.current.empty?
                            else
                                commands.append Ractor.receive
                            end

                            unless commands.empty?
                                commands.each { |command| in_queue.enqueue(command) }
                            end

                            if commands.any? { |cmd| cmd.is_a?(StopCommand) }
                                run = false
                                in_queue.enqueue(StopCommand.new)
                                break
                            end

                        end
                    end
                end
                break unless run
            end
        end

        @iterations.times do |i|
            ractor.send(TimeCommand.new(i + 1))
        end
        ractor.send(StopCommand.new)
        ractor.join
    end

    def run2
        ractor = Ractor.new do
            ractors = []
            loop do
                command = Ractor.receive
                if command.is_a?(StopCommand)
                    break
                else
                    ractors << Ractor.new(command) do |cmd|
                        cmd.task
                    end
                end
            end
            ractors.each(&:join)
        end

        @iterations.times do |i|
            ractor.send(TimeCommand.new(i + 1))
        end
        ractor.send(StopCommand.new)
        ractor.join
    end
end

worker = Worker.new(iterations: 10000)

p [:fibers, Benchmark.realtime { worker.run1 }.round(2)]
p [:ractors, Benchmark.realtime { worker.run2 }.round(2)]
```

**Output:**

Benchmark result:
```
[:fibers, 6.29]
[:ractors, 58.73]
```
The results show a nearly 10‑fold performance difference. This benchmark demonstrates why the Worker Ractor uses a pooled design with Fibers instead of spawning a Ractor per task. But, for merging ractors with fibers need to add ```Ractor#empty?``` method.

----------------------------------------
Feature #21930: Add Ractor#empty? method to check for pending messages without blocking
https://bugs.ruby-lang.org/issues/21930#change-116573

* Author: synacker (Mikhail Milovidov)
* Status: Feedback
----------------------------------------
**Summary**
In concurrent Ractor‑based architectures, there’s a critical need to check whether a Ractor has pending messages without blocking. Currently, this is not possible with the standard API

**Motivation**
The Ractor API provides a powerful mechanism for communication between system OS threads. However, in high‑load systems that use cooperative multitasking, the current Ractor#receive method presents limitations:

* It blocks the current thread until a message arrives.

* It doesn’t offer a non‑blocking way to check the message queue.

* This makes it difficult to integrate Ractors with cooperative scheduling frameworks (e.g., Async, Fiber‑based systems).

As a result, developers must either:

* Accept thread blocking (hurting responsiveness).

* Implement complex workarounds with timeouts or auxiliary queues.

**Proposed solution**
Add Ractor#empty? to the Ractor API. The method should:

* Return true if there are no pending messages in the Ractor’s main queue.

* Return false if there is at least one message available for processing.

* Not block the calling thread under any circumstances.

* Be safe to call from any Ractor (including the current one).

**Demonstration code**
Below is a proof‑of‑concept showing how Ractor#empty? enables cooperative multitasking with the Async gem:

```ruby
require 'async'

class TimeCommand
    attr_reader :id

    def initialize(id)
        @id = id
    end

    def task
        1.upto(3) do |i|      
            sleep(1)
            puts "[cmd #{@id}] step #{i} @ #{Time.now}"
        end
    end
end

class Worker
    def initialize
        @ractor = Ractor.new do
            loop do
                Sync do |task|
                    in_queue = Async::Queue.new
                    queue_task = task.async do |subtask|
                        while command = in_queue.dequeue
                            subtask.async do |child_task|
                                command.task
                            end
                        end
                    end

                    task.async(transient: true) do |main_task|
                        loop do
                            commands = []
                            if queue_task.children? || !in_queue.empty?
                                main_task.yield
                                commands.append Ractor.receive while !Ractor.current.empty?
                            else
                                commands.append Ractor.receive
                            end

                            unless commands.empty?
                                puts "Worker received batch of #{commands.size} commands."
                                commands.each { |command| in_queue.enqueue(command) }
                            end
                        end
                    end
                end
            end

        end
    end

    def send(command)
        @ractor.send(command, move: true)
    end

    def wait
        @ractor.join
    end
end

worker = Worker.new

1000.times do |i|
    100.times do |j|
        worker.send TimeCommand.new(i * 10 + j)
    end
    sleep(1)
end

worker.wait
```

**Key observations:**
With Ractor#empty?, developers can:

* Integrate Ractors with cooperative multitasking frameworks (e.g., Async) more naturally.

* Avoid thread blocking when checking for incoming messages.

* Batch process messages efficiently (collect all pending messages in one go).

* Improve responsiveness in high‑concurrency scenarios by yielding control back to the scheduler when no work is available.

**Benefits**

* Enables better integration with modern Ruby concurrency tools.

* Reduces need for complex workarounds.

* Improves performance in message‑driven architectures.

* Maintains Ractor’s thread‑safety guarantees.

-- 
https://bugs.ruby-lang.org/