The patch titled
Subject: mm: vmscan: do not reclaim from lower zones if they are balanced
has been added to the -mm tree. Its filename is
mm-vmscan-do-not-reclaim-from-lower-zones-if-they-are-balanced.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-vmscan-do-not-reclaim-from-lower-zones-if-they-are-balanced.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-vmscan-do-not-reclaim-from-lower-zones-if-they-are-balanced.patch
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------------------------------------------------------
From: Mel Gorman <mgorman@xxxxxxx>
Subject: mm: vmscan: do not reclaim from lower zones if they are balanced
Historically kswapd scanned from DMA->Movable in the opposite direction to
the page allocator to avoid allocating behind kswapd direction of
progress. The fair zone allocation policy altered this in a non-obvious
manner.
Traditionally, the page allocator prefers to use the highest eligible
zones in order until the low watermarks are reached and then wakes kswapd.
Once kswapd is awake, it scans zones in the opposite direction so the
scanning lists on 64-bit look like this;
Page alloc Kswapd
---------- ------
Movable DMA
Normal DMA32
DMA32 Normal
DMA Movable
If kswapd scanned in the same direction as the page allocator then it is
possible that kswapd would proportionally reclaim the lower zones that
were never used as the page allocator was always allocating behind the
reclaim. This would work as follows
pgalloc hits Normal low wmark
kswapd reclaims Normal
kswapd reclaims DMA32
pgalloc hits Normal low wmark
kswapd reclaims Normal
kswapd reclaims DMA32
The introduction of the fair zone allocation policy fundamentally altered
this problem by interleaving between zones until the low watermark is
reached. There are at least two issues with this
o The page allocator can allocate behind kswapds progress
(scans/reclaims lower zone and fair zone allocation policy then uses
those pages)
o When the low watermark of the high zone is reached there may recently
allocated pages allocated from the lower zone but as kswapd scans
dma->highmem to the highest zone needing balancing it'll reclaim the
lower zone even if it was balanced.
Let N = high_wmark(Normal) + high_wmark(DMA32). Of the last N
allocations, some percentage will be allocated from Normal and some from
DMA32. The percentage depends on the ratio of the zone sizes and when
their watermarks were hit. If Normal is unbalanced, DMA32 will be shrunk
by kswapd. If DMA32 is unbalanced only DMA32 will be shrunk. This leads
to a difference of ages between DMA32 and Normal. Relatively young pages
are then continually rotated and reclaimed from DMA32 due to the higher
zone being unbalanced. Some of these pages may be recently read-ahead
pages requiring that the page be re-read from disk and impacting overall
performance.
The problem is fundamental to the fact we have per-zone LRU and allocation
policies and ideally we would only have per-node allocation and LRU lists.
This would avoid the need for the fair zone allocation policy but the
low-memory-starvation issue would have to be addressed again from scratch.
kswapd shrinks equally from all zones up to the high watermark plus a
balance gap and the lowmem reserves. This patch removes the additional
reclaim from lower zones on the grounds that the fair zone allocation
policy will typically be interleaving between the zones. This should not
break the normal page aging as the proportional allocations due to the
fair zone allocation policy should compensate.
tiobench was used to evaluate this because it includes a simple sequential
reader which is the most obvious regression. It also has threaded readers
that produce reasonably steady figures.
3.16.0-rc2 3.0.0 3.16.0-rc2
vanilla vanilla checklow-v4
Min SeqRead-MB/sec-1 120.92 ( 0.00%) 133.65 ( 10.53%) 140.64 ( 16.31%)
Min SeqRead-MB/sec-2 100.25 ( 0.00%) 121.74 ( 21.44%) 117.67 ( 17.38%)
Min SeqRead-MB/sec-4 96.27 ( 0.00%) 113.48 ( 17.88%) 107.56 ( 11.73%)
Min SeqRead-MB/sec-8 83.55 ( 0.00%) 97.87 ( 17.14%) 88.08 ( 5.42%)
Min SeqRead-MB/sec-16 66.77 ( 0.00%) 82.59 ( 23.69%) 71.04 ( 6.40%)
There are still regressions for higher number of threads but this is
related to changes in the CFQ IO scheduler.
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/swap.h | 9 --------
mm/vmscan.c | 46 ++++++++++++++---------------------------
2 files changed, 16 insertions(+), 39 deletions(-)
diff -puN include/linux/swap.h~mm-vmscan-do-not-reclaim-from-lower-zones-if-they-are-balanced include/linux/swap.h
--- a/include/linux/swap.h~mm-vmscan-do-not-reclaim-from-lower-zones-if-they-are-balanced
+++ a/include/linux/swap.h
@@ -165,15 +165,6 @@ enum {
#define SWAP_CLUSTER_MAX 32UL
#define COMPACT_CLUSTER_MAX SWAP_CLUSTER_MAX
-/*
- * Ratio between zone->managed_pages and the "gap" that above the per-zone
- * "high_wmark". While balancing nodes, We allow kswapd to shrink zones that
- * do not meet the (high_wmark + gap) watermark, even which already met the
- * high_wmark, 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 KSWAPD_ZONE_BALANCE_GAP_RATIO 100
-
#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 -puN mm/vmscan.c~mm-vmscan-do-not-reclaim-from-lower-zones-if-they-are-balanced mm/vmscan.c
--- a/mm/vmscan.c~mm-vmscan-do-not-reclaim-from-lower-zones-if-they-are-balanced
+++ a/mm/vmscan.c
@@ -2307,7 +2307,7 @@ static unsigned long shrink_zone(struct
/* Returns true if compaction should go ahead for a high-order request */
static inline bool compaction_ready(struct zone *zone, int order)
{
- unsigned long balance_gap, watermark;
+ unsigned long watermark;
bool watermark_ok;
/*
@@ -2316,9 +2316,7 @@ static inline bool compaction_ready(stru
* there is a buffer of free pages available to give compaction
* a reasonable chance of completing and allocating the page
*/
- balance_gap = min(low_wmark_pages(zone), DIV_ROUND_UP(
- zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
- watermark = high_wmark_pages(zone) + balance_gap + (2UL << order);
+ watermark = high_wmark_pages(zone) + (2UL << order);
watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, 0, 0);
/*
@@ -2806,11 +2804,9 @@ static void age_active_anon(struct zone
} while (memcg);
}
-static bool zone_balanced(struct zone *zone, int order,
- unsigned long balance_gap, int classzone_idx)
+static bool zone_balanced(struct zone *zone, int order)
{
- if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone) +
- balance_gap, classzone_idx, 0))
+ if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone), 0, 0))
return false;
if (IS_ENABLED(CONFIG_COMPACTION) && order &&
@@ -2867,7 +2863,7 @@ static bool pgdat_balanced(pg_data_t *pg
continue;
}
- if (zone_balanced(zone, order, 0, i))
+ if (zone_balanced(zone, order))
balanced_pages += zone->managed_pages;
else if (!order)
return false;
@@ -2924,7 +2920,6 @@ static bool kswapd_shrink_zone(struct zo
unsigned long *nr_attempted)
{
int testorder = sc->order;
- unsigned long balance_gap;
struct reclaim_state *reclaim_state = current->reclaim_state;
struct shrink_control shrink = {
.gfp_mask = sc->gfp_mask,
@@ -2946,21 +2941,11 @@ static bool kswapd_shrink_zone(struct zo
testorder = 0;
/*
- * 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 = min(low_wmark_pages(zone), DIV_ROUND_UP(
- zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
-
- /*
* If there is no low memory pressure or the zone is balanced then no
* reclaim is necessary
*/
lowmem_pressure = (buffer_heads_over_limit && is_highmem(zone));
- if (!lowmem_pressure && zone_balanced(zone, testorder,
- balance_gap, classzone_idx))
+ if (!lowmem_pressure && zone_balanced(zone, testorder))
return true;
shrink_zone(zone, sc);
@@ -2983,7 +2968,7 @@ static bool kswapd_shrink_zone(struct zo
* waits.
*/
if (zone_reclaimable(zone) &&
- zone_balanced(zone, testorder, 0, classzone_idx)) {
+ zone_balanced(zone, testorder)) {
zone_clear_flag(zone, ZONE_CONGESTED);
zone_clear_flag(zone, ZONE_TAIL_LRU_DIRTY);
}
@@ -3069,7 +3054,7 @@ static unsigned long balance_pgdat(pg_da
break;
}
- if (!zone_balanced(zone, order, 0, 0)) {
+ if (!zone_balanced(zone, order)) {
end_zone = i;
break;
} else {
@@ -3114,12 +3099,13 @@ static unsigned long balance_pgdat(pg_da
/*
* Now scan the zone in the dma->highmem direction, stopping
- * at the last zone which needs scanning.
- *
- * We do this because the page allocator works in the opposite
- * direction. This prevents the page allocator from allocating
- * pages behind kswapd's direction of progress, which would
- * cause too much scanning of the lower zones.
+ * at the last zone which needs scanning. We do this because
+ * the page allocators prefers to work in the opposite
+ * direction and we want to avoid the page allocator reclaiming
+ * behind kswapd's direction of progress. Due to the fair zone
+ * allocation policy interleaving allocations between zones
+ * we no longer proportionally scan the lower zones if the
+ * watermarks are ok.
*/
for (i = 0; i <= end_zone; i++) {
struct zone *zone = pgdat->node_zones + i;
@@ -3387,7 +3373,7 @@ void wakeup_kswapd(struct zone *zone, in
}
if (!waitqueue_active(&pgdat->kswapd_wait))
return;
- if (zone_balanced(zone, order, 0, 0))
+ if (zone_balanced(zone, order))
return;
trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
_
Patches currently in -mm which might be from mgorman@xxxxxxx are
mm-page_alloc-fix-cma-area-initialisation-when-pageblock-max_order.patch
mm-page_alloc-add-__meminit-to-alloc_pages_exact_nid.patch
mm-thp-move-invariant-bug-check-out-of-loop-in-__split_huge_page_map.patch
mm-thp-replace-smp_mb-after-atomic_add-by-smp_mb__after_atomic.patch
mem-hotplug-improve-zone_movable_is_highmem-logic.patch
mm-vmscan-remove-remains-of-kswapd-managed-zone-all_unreclaimable.patch
mm-vmscan-rework-compaction-ready-signaling-in-direct-reclaim.patch
mm-vmscan-remove-all_unreclaimable.patch
mm-vmscan-move-swappiness-out-of-scan_control.patch
tracing-tell-mm_migrate_pages-event-about-numa_misplaced.patch
mm-export-nr_shmem-via-sysinfo2-si_meminfo-interfaces.patch
mm-pagemap-avoid-unnecessary-overhead-when-tracepoints-are-deactivated.patch
mm-rearrange-zone-fields-into-read-only-page-alloc-statistics-and-page-reclaim-lines.patch
mm-vmscan-do-not-reclaim-from-lower-zones-if-they-are-balanced.patch
mm-page_alloc-reduce-cost-of-the-fair-zone-allocation-policy.patch
mm-introduce-do_shared_fault-and-drop-do_fault-fix-fix.patch
mm-compactionc-isolate_freepages_block-small-tuneup.patch
mm-zbud-zbud_alloc-minor-param-change.patch
mm-zbud-change-zbud_alloc-size-type-to-size_t.patch
mm-zpool-implement-common-zpool-api-to-zbud-zsmalloc.patch
mm-zpool-zbud-zsmalloc-implement-zpool.patch
mm-zpool-update-zswap-to-use-zpool.patch
mm-zpool-prevent-zbud-zsmalloc-from-unloading-when-used.patch
do_shared_fault-check-that-mmap_sem-is-held.patch
linux-next.patch
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