A long, long time ago, in a CSRG far away, the page daemon, a system
process responsible for handling swap, was running on a fixed frequency.
Then, good ideas from the Mach VM were borrowed, and it was changed to
run "when there are good reasons to trigger it".

But the documentation of the counter of invocations/wakeups of that
daemon was left unchanged.

Which is why vmstat -s reports it as the number of "revolutions of the
clock hand", which someone might believe is something time-related, but
is nowadays completely different.

The following diff tries to put a bit more sense in this.

Index: sys/uvm/uvmexp.h
===================================================================
RCS file: /OpenBSD/src/sys/uvm/uvmexp.h,v
diff -u -p -r1.17 uvmexp.h
--- sys/uvm/uvmexp.h	25 Feb 2025 11:29:17 -0000	1.17
+++ sys/uvm/uvmexp.h	7 Mar 2025 15:33:56 -0000
@@ -139,7 +139,7 @@ struct uvmexp {
 
 	/* daemon counters */
 	int pdwoke;	/* [F] # of times daemon woke up */
-	int pdrevs;	/* number of times daemon rev'd clock hand */
+	int pdrevs;	/* number of times daemon called for free pages */
 	int pdswout;	/* number of times daemon called for swapout */
 	int pdfreed;	/* number of pages daemon freed since boot */
 	int pdscans;	/* number of pages daemon scanned since boot */
Index: usr.bin/vmstat/vmstat.c
===================================================================
RCS file: /OpenBSD/src/usr.bin/vmstat/vmstat.c,v
diff -u -p -r1.158 vmstat.c
--- usr.bin/vmstat/vmstat.c	1 May 2024 12:54:27 -0000	1.158
+++ usr.bin/vmstat/vmstat.c	7 Mar 2025 15:33:56 -0000
@@ -542,7 +542,7 @@ dosum(void)
 	/* daemon counters */
 	(void)printf("%11u number of times the pagedaemon woke up\n",
 		     uvmexp.pdwoke);
-	(void)printf("%11u revolutions of the clock hand\n", uvmexp.pdrevs);
+	(void)printf("%11u number of times the pagedaemon scanned for free pages\n", uvmexp.pdrevs);
 	(void)printf("%11u pages freed by pagedaemon\n", uvmexp.pdfreed);
 	(void)printf("%11u pages scanned by pagedaemon\n", uvmexp.pdscans);
 	(void)printf("%11u pages reactivated by pagedaemon\n", uvmexp.pdreact);