docs: Add notice about WindowsSelectorEventLoop on py38

Fixes #2608

diff --git a/docs/index.rst b/docs/index.rst
index 6d2e1d73c25ab75047cf19744603312bc6763d7b..135fd099ef84db38892f17db4f881b29edbcb49c 100644 (file)
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -120,6 +120,13 @@ is limited (Even though Tornado is built on ``asyncio``, which
 supports Windows, Tornado does not use the APIs that are necessary for
 scalable networking on Windows).
 
+On Windows, Tornado requires the ``WindowsSelectorEventLoop``. This is
+the default in Python 3.7 and older, but Python 3.8 defaults to an
+event loop that is not compatible with Tornado. Applications that use
+Tornado on Windows with Python 3.8 must call
+``asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())``
+at the beginning of their ``main`` file/function.
+
 Documentation
 -------------