[PATCH 3/3] Zone reclaim V3: Frequency of failed reclaim attempts

[Christoph Lameter]

Reduce frequency of unsuccessful zone reclaim attempts

It is unlikely that zone reclaim is successful once it has failed. The
performance of the page allocator will sink signficantly for off-node
allocation if every page allocation attempt first requires a zone reclaim
scan to establish that no local memory is availale.

This patch limits the number of unsuccessful zone reclaim attempts to one
per tick by remembering the last time a zone reclaim failed on a zone.

Note that this approach may be avoided once we have per zone statistics
on the number of unmapped (==easily reclaimable) pages. I am working on
a statistics patch that may allow keeping track of unmapped pages per
zone. A check of that number may then allow an easy determination if it
makes sense to run zone reclaim.

Signed-off-by: Christoph Lameter <clameter@xxxxxxx>

Index: linux-2.6.15-rc4/mm/vmscan.c
===================================================================
--- linux-2.6.15-rc4.orig/mm/vmscan.c	2005-12-08 11:10:14.000000000 -0800
+++ linux-2.6.15-rc4/mm/vmscan.c	2005-12-08 12:04:38.000000000 -0800
@@ -1379,6 +1379,16 @@ int zone_reclaim(struct zone *zone, gfp_
 	    atomic_read(&zone->reclaim_in_progress) > 0)
 		return 0;
 
+	/*
+	 * If an unsuccessful zone reclaim occurred in this tick then we
+	 * already needed to go off before. Our local purity is already
+	 * tainted and its likely that the scan for easily reclaimable pages
+	 * will be a waste of time. Continue off node allocations for the
+	 * duration of this tick.
+	 */
+	if (zone->last_unsuccessful_zone_reclaim == get_jiffies_64())
+		return 0;
+
 	sc.gfp_mask = gfp_mask;
 	sc.may_writepage = 0;
 	sc.may_swap = 0;
@@ -1397,6 +1407,8 @@ int zone_reclaim(struct zone *zone, gfp_
 	shrink_zone(zone, &sc);
 	p->reclaim_state = NULL;
 	current->flags &= ~PF_MEMALLOC;
+	if (sc.nr_reclaimed == 0)
+		zone->last_unsuccessful_zone_reclaim = get_jiffies_64();
 	cond_resched();
 	return sc.nr_reclaimed >= (1 << order);
 }
Index: linux-2.6.15-rc4/include/linux/mmzone.h
===================================================================
--- linux-2.6.15-rc4.orig/include/linux/mmzone.h	2005-12-08 11:10:14.000000000 -0800
+++ linux-2.6.15-rc4/include/linux/mmzone.h	2005-12-08 12:00:43.000000000 -0800
@@ -153,6 +153,8 @@ struct zone {
 	/* A count of how many reclaimers are scanning this zone */
 	atomic_t		reclaim_in_progress;
 
+	unsigned long		last_unsuccessful_zone_reclaim;
+
 	/*
 	 * prev_priority holds the scanning priority for this zone.  It is
 	 * defined as the scanning priority at which we achieved our reclaim
-
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