- Add an event that is triggered when the system is low on memory. This is

intended to be used by significant memory consumers so that they may drain
   some of their caches.

Inspired by:	phk
Approved by:	re
Tested on:	x86, alpha

sys/sys/eventhandler.h

@@ -179,5 +179,9 @@ typedef void (*idle_eventhandler_t)(void *, int);
 #define IDLE_PRI_LAST		20000
 EVENTHANDLER_FAST_DECLARE(idle_event, idle_eventhandler_t);
 
+/* Low memory event */
+typedef void (*vm_lowmem_handler_t)(void *, int);
+#define	LOWMEM_PRI_DEFAULT	0
+EVENTHANDLER_DECLARE(vm_lowmem, vm_lowmem_handler_t);
 
 #endif /* SYS_EVENTHANDLER_H */

sys/vm/vm_pageout.c

@@ -76,6 +76,7 @@
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/kernel.h>
+#include <sys/eventhandler.h>
 #include <sys/lock.h>
 #include <sys/mutex.h>
 #include <sys/proc.h>
@@ -677,11 +678,18 @@ vm_pageout_scan(int pass)
 	struct thread *td;
 
 	GIANT_REQUIRED;
+	/*
+	 * Decrease registered cache sizes.
+	 */
+	EVENTHANDLER_INVOKE(vm_lowmem, 0);
+	/*
+	 * We do this explicitly after the caches have been drained above.
+	 */
+	uma_reclaim();
 	/*
 	 * Do whatever cleanup that the pmap code can.
 	 */
 	vm_pageout_pmap_collect();
-	uma_reclaim();
 
 	addl_page_shortage_init = vm_pageout_deficit;
 	vm_pageout_deficit = 0;