add doc for cub_enabled
@@ -26,3 +26,16 @@ These functions cover a subset of
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+
+
+CUB backend for reduction routines
+----------------------------------
+Some CuPy reduction routines, including :func:`~cupy.sum`, :func:`~cupy.min`, :func:`~cupy.max`,
+:func:`~cupy.argmin`, :func:`~cupy.argmax`, and other functions built on top of them, can be
+accelerated by switching to the `CUB backend`_. The switch can be toggled on or off at runtime
+by setting the bool :data:`cupy.cuda.cub_enabled`, which is set to ``False`` by default. Note
+that while in general CUB-backed reductions are faster, there could be exceptions depending on
+the data layout. We recommend users to perform some benchmarks to determine whether CUB offers
+better performance or not.
+
+.. _CUB backend: http://nvlabs.github.io/cub/
