diff --git a/doc/example.conf b/doc/example.conf
index b96d3c986..14ce60bd7 100644
--- a/doc/example.conf
+++ b/doc/example.conf
@@ -41,10 +41,12 @@ server:
 	# msg-cache-size: 4194304
 
 	# the number of slabs to use for the message cache.
+	# the number of slabs must be a power of 2.
 	# more slabs reduce lock contention, but fragment memory usage.
 	# msg-cache-slabs: 4
 
 	# the number of queries that a thread gets to service.
+	# this must be a power of 2.
 	# num-queries-per-thread: 1024
 
 	# Enable IPv4, "yes" or "no".