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Date:      Fri, 26 Jul 2013 23:22:05 +0000 (UTC)
From:      Jeff Roberson <jeff@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r253697 - head/sys/vm
Message-ID:  <201307262322.r6QNM51H095113@svn.freebsd.org>
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Author: jeff
Date: Fri Jul 26 23:22:05 2013
New Revision: 253697
URL: http://svnweb.freebsd.org/changeset/base/253697

Log:
  Improve page LRU quality and simplify the logic.
  
   - Don't short-circuit aging tests for unmapped objects.  This biases
     against unmapped file pages and transient mappings.
   - Always honor PGA_REFERENCED.  We can now use this after soft busying
     to lazily restart the LRU.
   - Don't transition directly from active to cached bypassing the inactive
     queue.  This frees recently used data much too early.
   - Rename actcount to act_delta to be more consistent with use and meaning.
  
  Reviewed by:	kib, alc
  Sponsored by:	EMC / Isilon Storage Division

Modified:
  head/sys/vm/vm_pageout.c

Modified: head/sys/vm/vm_pageout.c
==============================================================================
--- head/sys/vm/vm_pageout.c	Fri Jul 26 22:53:17 2013	(r253696)
+++ head/sys/vm/vm_pageout.c	Fri Jul 26 23:22:05 2013	(r253697)
@@ -708,7 +708,7 @@ vm_pageout_object_deactivate_pages(pmap_
 {
 	vm_object_t backing_object, object;
 	vm_page_t p;
-	int actcount, remove_mode;
+	int act_delta, remove_mode;
 
 	VM_OBJECT_ASSERT_LOCKED(first_object);
 	if ((first_object->flags & OBJ_FICTITIOUS) != 0)
@@ -739,17 +739,17 @@ vm_pageout_object_deactivate_pages(pmap_
 				vm_page_unlock(p);
 				continue;
 			}
-			actcount = pmap_ts_referenced(p);
+			act_delta = pmap_ts_referenced(p);
 			if ((p->aflags & PGA_REFERENCED) != 0) {
-				if (actcount == 0)
-					actcount = 1;
+				if (act_delta == 0)
+					act_delta = 1;
 				vm_page_aflag_clear(p, PGA_REFERENCED);
 			}
-			if (p->queue != PQ_ACTIVE && actcount != 0) {
+			if (p->queue != PQ_ACTIVE && act_delta != 0) {
 				vm_page_activate(p);
-				p->act_count += actcount;
+				p->act_count += act_delta;
 			} else if (p->queue == PQ_ACTIVE) {
-				if (actcount == 0) {
+				if (act_delta == 0) {
 					p->act_count -= min(p->act_count,
 					    ACT_DECLINE);
 					if (!remove_mode && p->act_count == 0) {
@@ -869,7 +869,7 @@ vm_pageout_scan(int pass)
 	int page_shortage, maxscan, pcount;
 	int addl_page_shortage;
 	vm_object_t object;
-	int actcount;
+	int act_delta;
 	int vnodes_skipped = 0;
 	int maxlaunder;
 	boolean_t queues_locked;
@@ -989,44 +989,40 @@ vm_pageout_scan(int pass)
 		queues_locked = FALSE;
 
 		/*
-		 * If the object is not being used, we ignore previous 
+		 * We bump the activation count if the page has been
+		 * referenced while in the inactive queue.  This makes
+		 * it less likely that the page will be added back to the
+		 * inactive queue prematurely again.  Here we check the 
+		 * page tables (or emulated bits, if any), given the upper 
+		 * level VM system not knowing anything about existing 
 		 * references.
 		 */
-		if (object->ref_count == 0) {
+		act_delta = 0;
+		if ((m->aflags & PGA_REFERENCED) != 0) {
 			vm_page_aflag_clear(m, PGA_REFERENCED);
+			act_delta = 1;
+		}
+		if (object->ref_count != 0) {
+			act_delta += pmap_ts_referenced(m);
+		} else {
 			KASSERT(!pmap_page_is_mapped(m),
 			    ("vm_pageout_scan: page %p is mapped", m));
-
-		/*
-		 * Otherwise, if the page has been referenced while in the 
-		 * inactive queue, we bump the "activation count" upwards, 
-		 * making it less likely that the page will be added back to 
-		 * the inactive queue prematurely again.  Here we check the 
-		 * page tables (or emulated bits, if any), given the upper 
-		 * level VM system not knowing anything about existing 
-		 * references.
-		 */
-		} else if ((m->aflags & PGA_REFERENCED) == 0 &&
-		    (actcount = pmap_ts_referenced(m)) != 0) {
-			vm_page_activate(m);
-			VM_OBJECT_WUNLOCK(object);
-			m->act_count += actcount + ACT_ADVANCE;
-			vm_page_unlock(m);
-			goto relock_queues;
 		}
 
 		/*
 		 * If the upper level VM system knows about any page 
-		 * references, we activate the page.  We also set the 
-		 * "activation count" higher than normal so that we will less 
-		 * likely place pages back onto the inactive queue again.
+		 * references, we reactivate the page or requeue it.
 		 */
-		if ((m->aflags & PGA_REFERENCED) != 0) {
-			vm_page_aflag_clear(m, PGA_REFERENCED);
-			actcount = pmap_ts_referenced(m);
-			vm_page_activate(m);
+		if (act_delta != 0) {
+			if (object->ref_count) {
+				vm_page_activate(m);
+				m->act_count += act_delta + ACT_ADVANCE;
+			} else {
+				vm_pagequeue_lock(pq);
+				queues_locked = TRUE;
+				vm_page_requeue_locked(m);
+			}
 			VM_OBJECT_WUNLOCK(object);
-			m->act_count += actcount + ACT_ADVANCE + 1;
 			vm_page_unlock(m);
 			goto relock_queues;
 		}
@@ -1324,50 +1320,40 @@ relock_queues:
 		/*
 		 * Check to see "how much" the page has been used.
 		 */
-		actcount = 0;
-		if (object->ref_count != 0) {
-			if (m->aflags & PGA_REFERENCED) {
-				actcount += 1;
-			}
-			actcount += pmap_ts_referenced(m);
-			if (actcount) {
-				m->act_count += ACT_ADVANCE + actcount;
-				if (m->act_count > ACT_MAX)
-					m->act_count = ACT_MAX;
-			}
+		act_delta = 0;
+		if (m->aflags & PGA_REFERENCED) {
+			vm_page_aflag_clear(m, PGA_REFERENCED);
+			act_delta += 1;
 		}
+		if (object->ref_count != 0)
+			act_delta += pmap_ts_referenced(m);
 
 		/*
-		 * Since we have "tested" this bit, we need to clear it now.
+		 * Advance or decay the act_count based on recent usage.
 		 */
-		vm_page_aflag_clear(m, PGA_REFERENCED);
+		if (act_delta) {
+			m->act_count += ACT_ADVANCE + act_delta;
+			if (m->act_count > ACT_MAX)
+				m->act_count = ACT_MAX;
+		} else {
+			m->act_count -= min(m->act_count, ACT_DECLINE);
+			act_delta = m->act_count;
+		}
 
 		/*
-		 * Only if an object is currently being used, do we use the
-		 * page activation count stats.
+		 * Move this page to the tail of the active or inactive
+		 * queue depending on usage.
 		 */
-		if (actcount != 0 && object->ref_count != 0)
+		if (act_delta == 0) {
+			KASSERT(object->ref_count != 0 ||
+			    !pmap_page_is_mapped(m),
+			    ("vm_pageout_scan: page %p is mapped", m));
+			/* Dequeue to avoid later lock recursion. */
+			vm_page_dequeue_locked(m);
+			vm_page_deactivate(m);
+			page_shortage--;
+		} else
 			vm_page_requeue_locked(m);
-		else {
-			m->act_count -= min(m->act_count, ACT_DECLINE);
-			if (object->ref_count == 0 ||
-			    m->act_count == 0) {
-				page_shortage--;
-				/* Dequeue to avoid later lock recursion. */
-				vm_page_dequeue_locked(m);
-				if (object->ref_count == 0) {
-					KASSERT(!pmap_page_is_mapped(m),
-				    ("vm_pageout_scan: page %p is mapped", m));
-					if (m->dirty == 0)
-						vm_page_cache(m);
-					else
-						vm_page_deactivate(m);
-				} else {
-					vm_page_deactivate(m);
-				}
-			} else
-				vm_page_requeue_locked(m);
-		}
 		vm_page_unlock(m);
 		VM_OBJECT_WUNLOCK(object);
 		m = next;

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