diff --git a/docs/users_guide/9.10.1-notes.rst b/docs/users_guide/9.10.1-notes.rst
index e5b80043bdf7b77efac6702f92793443d6e66a5f..8a3c9db4b0885f3e7c0400438839f7c5fff2bdf8 100644
--- a/docs/users_guide/9.10.1-notes.rst
+++ b/docs/users_guide/9.10.1-notes.rst
@@ -60,6 +60,11 @@ GHCi
 Runtime system
 ~~~~~~~~~~~~~~
 
+- Internal fragmentation incurred by the non-moving GC's allocator has been reduced for small objects.
+  In one real-world application, this has reduced resident set size by about 20% and modestly improved run-time.
+  See :ghc-ticket:`23340`.
+  :rts-flag:`--nonmoving-dense-allocator-count=⟨count⟩` has been added to fine-tune this behaviour.
+
 ``base`` library
 ~~~~~~~~~~~~~~~~