Add note in `free_all_blocks` reference
@@ -1277,6 +1277,11 @@ cdef class MemoryPool(object):
stream (cupy.cuda.Stream): Release free blocks in the arena
of the given stream. The default releases blocks in all
arenas.
+
+ .. note::
+ A memory pool may split a free block for space efficiency. A split
+ block is not released until all its parts are merged back into one
+ even if :meth:`free_all_blocks` is called.
"""
mp = <SingleDeviceMemoryPool>self._pools[device.get_device_id()]
mp.free_all_blocks(stream=stream)
