diff --git a/Doc/library/asyncio-threading.rst b/Doc/library/asyncio-threading.rst new file mode 100644 index 000000000000000..baeb50e9cad472d --- /dev/null +++ b/Doc/library/asyncio-threading.rst @@ -0,0 +1,142 @@ +.. currentmodule:: asyncio + +.. _asyncio-threading: + +asyncio and free-threaded Python +================================ + +asyncio uses an event loop as a scheduler to enable highly efficient +I/O-bound concurrency by switching between tasks during non-blocking I/O +operations. It allows off-loading CPU-bound work to a thread or process +pool, but that is still limited by the :term:`global interpreter lock` +in CPython. + +However, with :pep:`703`, Python can now be built in a "free-threaded" +mode, which removes the GIL and allows true multi-threading. This means +that asyncio can now take advantage of multiple CPU cores without the +limitations imposed by the GIL. + +Since Python 3.14, asyncio has first-class support for free-threaded +Python, and the implementation of asyncio is safe to use in a +multi-threaded environment. + +.. seealso:: + + `Scaling asyncio on Free-Threaded Python + `__, + a blog post by Kumar Aditya which explains the internal changes + that make asyncio safe and efficient under free-threaded Python, + together with benchmarks of the resulting improvements. + + +Thread safety considerations +---------------------------- + +While asyncio is designed to be thread-safe in a free-threaded Python +environment, there are still some considerations to keep in mind when +using asyncio with threads: + +1. **Event loop**: Each thread should have its own event loop which + should not be shared across threads. This ensures that the event loop + can manage its own tasks and callbacks without interference from + other threads. + +2. **Task management**: Tasks and futures created in one thread should + not be awaited or manipulated from another thread. + +3. **Thread-safe APIs**: When interacting with asyncio from multiple + threads, it's important to use thread-safe APIs provided by asyncio, + such as :func:`asyncio.run_coroutine_threadsafe` for submitting + coroutines to an event loop from another thread. If you need to + call a callback from a different thread, you can use + :meth:`loop.call_soon_threadsafe` to schedule it safely. + +4. **Synchronization**: The synchronization primitives provided by + asyncio (like :class:`asyncio.Lock` and :class:`asyncio.Event`) + are not designed to be used across threads. If you need to + synchronize between threads, you should use the synchronization + primitives from the :mod:`threading` module instead. + + +Using asyncio with threads +-------------------------- + +asyncio supports running one event loop per thread, which allows you to +take advantage of multiple CPU cores in a free-threaded Python +environment. Each thread can run its own event loop, and tasks can be +scheduled on those loops independently. + +Here's an example of how to use asyncio with threads:: + + import asyncio + import threading + + async def worker(name: str) -> None: + print(f"Worker {name} starting") + await asyncio.sleep(1) + print(f"Worker {name} done") + + def run_loop(name: str) -> None: + asyncio.run(worker(name)) + + threads = [ + threading.Thread(target=run_loop, args=(f"T{i}",)) + for i in range(4) + ] + for t in threads: + t.start() + for t in threads: + t.join() + +In this example, each thread creates its own event loop with +:func:`asyncio.run` and runs a coroutine on it. The threads execute +concurrently, and in a free-threaded build they can run on separate +CPU cores in parallel. + + +Producer/consumer across threads +-------------------------------- + +When a regular (non-asyncio) thread needs to hand work to an asyncio +event loop running in another thread, use a thread-safe primitive such +as :class:`queue.Queue` rather than :class:`asyncio.Queue`, which is +only safe within a single event loop.:: + + import asyncio + import queue + import threading + + def producer(q: queue.Queue[int]) -> None: + for i in range(5): + print(f"Producing {i}") + q.put(i) + q.shutdown() + + async def consumer(q: queue.Queue[int]) -> None: + while True: + try: + item = q.get_nowait() + except queue.Empty: + await asyncio.sleep(0.1) + continue + except queue.ShutDown: + break + print(f"Consumed {item}") + await asyncio.sleep(item) + + q: queue.Queue[int] = queue.Queue() + consumer_thread = threading.Thread( + target=lambda: asyncio.run(consumer(q)) + ) + consumer_thread.start() + producer(q) + consumer_thread.join() + +The producer runs on the main thread while the consumer runs inside an +event loop on its own thread, yet they communicate safely through +``queue.Queue``. When the queue is empty the consumer sleeps briefly +and tries again. When the producer is done it calls +:meth:`~queue.Queue.shutdown`, which causes subsequent +:meth:`~queue.Queue.get_nowait` calls to raise :exc:`queue.ShutDown` +so the consumer can exit cleanly. + diff --git a/Doc/library/asyncio.rst b/Doc/library/asyncio.rst index 0f72e31dee5f1d1..90a465f3e1d3af4 100644 --- a/Doc/library/asyncio.rst +++ b/Doc/library/asyncio.rst @@ -128,6 +128,7 @@ for full functionality and the latest features. asyncio-api-index.rst asyncio-llapi-index.rst asyncio-dev.rst + asyncio-threading.rst .. note:: The source code for asyncio can be found in :source:`Lib/asyncio/`.