From: Martin Pieuchot <mpi@grenadille.net>
Subject: OOM, page daemon & responsiveness
To: tech@openbsd.org
Cc: sthen@openbsd.org
Date: Mon, 30 Sep 2024 10:39:55 +0200

I observed that the "freezes" happening when the page daemon is freeing
memory are almost always due to a large scan of inactive pages.  During
such scans, the kernel lock is held and most processes are waiting for
physical pages.

The diff below reduces the duration of such scans by removing a change
which wasn't present in 4.4BSD VM's and brings back coherency regarding
free pages watermarks.

This diff makes the page daemon stop scanning as soon as the high water
mark has been reached.

Even if this might not be considered a real fix and we could do much
better by adapting the logic to today's amount of RAM, it is a step in
such direction because it brings back the original & coherent logic.

ok?

Index: uvm/uvm_pdaemon.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_pdaemon.c,v
diff -u -p -r1.115 uvm_pdaemon.c
--- uvm/uvm_pdaemon.c	30 Sep 2024 08:09:39 -0000	1.115
+++ uvm/uvm_pdaemon.c	30 Sep 2024 08:17:00 -0000
@@ -468,7 +468,7 @@ uvmpd_scan_inactive(struct uvm_pmalloc *
 			 */
 			free = uvmexp.free - BUFPAGES_DEFICIT;
 			if (((pma == NULL || (pma->pm_flags & UVM_PMA_FREED)) &&
-			    (free + uvmexp.paging >= uvmexp.freetarg << 2)) ||
+			    (free + uvmexp.paging >= uvmexp.freetarg)) ||
 			    dirtyreacts == UVMPD_NUMDIRTYREACTS) {
 				if (swslot == 0) {
 					/* exit now if no swap-i/o pending */
@@ -565,7 +565,7 @@ uvmpd_scan_inactive(struct uvm_pmalloc *
 			 * free target when all the current pageouts complete.
 			 */
 			if ((pma == NULL || (pma->pm_flags & UVM_PMA_FREED)) &&
-			    (free + uvmexp.paging > uvmexp.freetarg << 2)) {
+			    (free + uvmexp.paging > uvmexp.freetarg)) {
 				rw_exit(slock);
 				continue;
 			}