diff --git a/synapse/config/server.py b/synapse/config/server.py
index 18102656b0..2a5918f48d 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -209,6 +209,8 @@ class ServerConfig(Config):
         # different cores. See
         # https://www.mirantis.com/blog/improve-performance-python-programs-restricting-single-cpu/.
         #
+        # This setting requires the affinity package to be installed!
+        #
         # cpu_affinity: 0xFFFFFFFF
 
         # Whether to serve a web client from the HTTP/HTTPS root resource.