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Old 03-22-2011, 10:33 PM
Mel Gorman
Default mm: vmscan: kswapd should not free an excessive numb=

BugLink: http://bugs.launchpad.net/bugs/808509

When reclaiming for order-0 pages, kswapd requires that all zones be
balanced. Each cycle through balance_pgdat() does background ageing on
all zones if necessary and applies equal pressure on the inactive zone
unless a lot of pages are free already.

A "lot of free pages" is defined as a "balance gap" above the high
watermark which is currently 7*high_watermark. Historically this was
reasonable as min_free_kbytes was small. However, on systems using huge
pages, it is recommended that min_free_kbytes is higher and it is tuned
with hugeadm --set-recommended-min_free_kbytes. With the introduction of
transparent huge page support, this recommended value is also applied. O=
X86-64 with 4G of memory, min_free_kbytes becomes 67584 so one would
expect around 68M of memory to be free. The Normal zone is approximately
35000 pages so under even normal memory pressure such as copying a large
file, it gets exhausted quickly. As it is getting exhausted, kswapd
applies pressure equally to all zones, including the DMA32 zone. DMA32 i=
approximately 700,000 pages with a high watermark of around 23,000 pages.
In this situation, kswapd will reclaim around (23000*8 where 8 is the hig=
watermark + balance gap of 7 * high watermark) pages or 718M of pages
before the zone is ignored. What the user sees is that free memory far
higher than it should be.

To avoid an excessive number of pages being reclaimed from the larger
zones, explicitely defines the "balance gap" to be either 1% of the zone
or the low watermark for the zone, whichever is smaller. While kswapd
will check all zones to apply pressure, it'll ignore zones that meets the
(high_wmark + balance_gap) watermark.

To test this, 80G were copied from a partition and the amount of memory
being used was recorded. A comparison of a patch and unpatched kernel ca=
be seen at
and shows that kswapd is not reclaiming as much memory with the patch

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Shaohua Li <shaohua.li@intel.com>
Cc: "Chen, Tim C" <tim.c.chen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 8afdcece4911e51cfff2b50a269418914cab8a3f)

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
include/linux/swap.h | 9 +++++++++
mm/vmscan.c | 16 +++++++++++++---
2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 97a5514..3b454c0 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -155,6 +155,15 @@ enum {
+ * Ratio between the present memory in the zone and the "gap" that
+ * we're allowing kswapd to shrink in addition to the per-zone high
+ * wmark, even for zones that already have the high wmark satisfied,
+ * in order to provide better per-zone lru behavior. We are ok to
+ * spend not more than 1% of the memory for this zone balancing "gap".
+ */
#define SWAP_MAP_MAX 0x3e /* Max duplication count, in first swap_map */
#define SWAP_MAP_BAD 0x3f /* Note pageblock is bad, in first swap_map */
#define SWAP_HAS_CACHE 0x40 /* Flag page is cached, in first swap_map */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a578535..f50f716 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2406,6 +2406,7 @@ loop_again:
for (i =3D 0; i <=3D end_zone; i++) {
struct zone *zone =3D pgdat->node_zones + i;
int nr_slab;
+ unsigned long balance_gap;
if (!populated_zone(zone))
@@ -2422,11 +2423,20 @@ loop_again:
mem_cgroup_soft_limit_reclaim(zone, order, sc.gfp_mask);
- * We put equal pressure on every zone, unless one
- * zone has way too many pages free already.
+ * We put equal pressure on every zone, unless
+ * one zone has way too many pages free
+ * already. The "too many pages" is defined
+ * as the high wmark plus a "gap" where the
+ * gap is either the low watermark or 1%
+ * of the zone, whichever is smaller.
+ balance_gap =3D min(low_wmark_pages(zone),
+ (zone->present_pages +
if (!zone_watermark_ok_safe(zone, order,
- 8*high_wmark_pages(zone), end_zone, 0))
+ high_wmark_pages(zone) + balance_gap,
+ end_zone, 0))
shrink_zone(priority, zone, &sc);
reclaim_state->reclaimed_slab =3D 0;
nr_slab =3D shrink_slab(sc.nr_scanned, GFP_KERNEL,

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