Re: [patch v2 3/3] mm: page_alloc: fair zone allocator policy
From: Johannes Weiner
Date: Thu Aug 08 2013 - 00:16:57 EST
On Wed, Aug 07, 2013 at 11:37:43AM -0400, Johannes Weiner wrote:
> On Wed, Aug 07, 2013 at 03:58:28PM +0100, Mel Gorman wrote:
> > On Fri, Aug 02, 2013 at 11:37:26AM -0400, Johannes Weiner wrote:
> > > @@ -352,6 +352,7 @@ struct zone {
> > > * free areas of different sizes
> > > */
> > > spinlock_t lock;
> > > + int alloc_batch;
> > > int all_unreclaimable; /* All pages pinned */
> > > #if defined CONFIG_COMPACTION || defined CONFIG_CMA
> > > /* Set to true when the PG_migrate_skip bits should be cleared */
> >
> > This adds a dirty cache line that is updated on every allocation even if
> > it's from the per-cpu allocator. I am concerned that this will introduce
> > noticable overhead in the allocator paths on large machines running
> > allocator intensive workloads.
> >
> > Would it be possible to move it into the per-cpu pageset? I understand
> > that hte round-robin nature will then depend on what CPU is running and
> > the performance characterisics will be different. There might even be an
> > adverse workload that uses all the batches from all available CPUs until
> > it is essentially the same problem but that would be a very worst case.
> > I would hope that in general it would work without adding a big source of
> > dirty cache line bouncing in the middle of the allocator.
>
> Rik made the same suggestion. The per-cpu error is one thing, the
> problem is if the allocating task and kswapd run on the same CPU and
> bypass the round-robin allocator completely, at which point we are
> back to square one. We'd have to reduce the per-cpu lists from a pool
> to strict batching of frees and allocs without reuse in between. That
> might be doable, I'll give this another look.
I found a way. It's still in the fast path, but I'm using vmstat
percpu counters and can stick the update inside the same irq-safe
section that does the other statistic updates.
On a two socket system with a small Normal zone, the results are as
follows (unfair: mmotm without the fairness allocator, fairpcp: the
fair allocator + the vmstat optimization):
---
pft
mmotm mmotm
unfair fairpcp
User 1 0.0258 ( 0.00%) 0.0254 ( 1.40%)
User 2 0.0264 ( 0.00%) 0.0263 ( 0.21%)
User 3 0.0271 ( 0.00%) 0.0277 ( -2.36%)
User 4 0.0287 ( 0.00%) 0.0280 ( 2.33%)
System 1 0.4904 ( 0.00%) 0.4919 ( -0.29%)
System 2 0.6141 ( 0.00%) 0.6183 ( -0.68%)
System 3 0.7346 ( 0.00%) 0.7349 ( -0.04%)
System 4 0.8700 ( 0.00%) 0.8704 ( -0.05%)
Elapsed 1 0.5164 ( 0.00%) 0.5182 ( -0.35%)
Elapsed 2 0.3213 ( 0.00%) 0.3235 ( -0.67%)
Elapsed 3 0.2800 ( 0.00%) 0.2800 ( 0.00%)
Elapsed 4 0.2304 ( 0.00%) 0.2303 ( 0.01%)
Faults/cpu 1 392724.3239 ( 0.00%) 391558.5131 ( -0.30%)
Faults/cpu 2 319357.5074 ( 0.00%) 317577.8745 ( -0.56%)
Faults/cpu 3 265703.1420 ( 0.00%) 265668.3579 ( -0.01%)
Faults/cpu 4 225516.0058 ( 0.00%) 225474.1508 ( -0.02%)
Faults/sec 1 392051.3043 ( 0.00%) 390880.8201 ( -0.30%)
Faults/sec 2 635360.7045 ( 0.00%) 631819.1117 ( -0.56%)
Faults/sec 3 725535.2889 ( 0.00%) 725525.1280 ( -0.00%)
Faults/sec 4 883307.5047 ( 0.00%) 884026.1721 ( 0.08%)
The overhead appears to be negligible, if not noise.
mmotm mmotm
unfair fairpcp
User 39.90 39.70
System 1070.93 1069.50
Elapsed 557.47 556.86
mmotm mmotm
unfair fairpcp
Page Ins 1248 876
Page Outs 4280 4184
Swap Ins 0 0
Swap Outs 0 0
Alloc DMA 0 0
Alloc DMA32 13098002 214383756
Alloc Normal 279611269 78332806
Alloc Movable 0 0
Direct pages scanned 0 0
Kswapd pages scanned 0 0
Kswapd pages reclaimed 0 0
Direct pages reclaimed 0 0
Kswapd efficiency 100% 100%
Kswapd velocity 0.000 0.000
Direct efficiency 100% 100%
Direct velocity 0.000 0.000
Percentage direct scans 0% 0%
Page writes by reclaim 0 0
Page writes file 0 0
Page writes anon 0 0
Page reclaim immediate 0 0
Page rescued immediate 0 0
Slabs scanned 0 0
Direct inode steals 0 0
Kswapd inode steals 0 0
Kswapd skipped wait 0 0
THP fault alloc 0 0
THP collapse alloc 0 0
THP splits 0 0
THP fault fallback 0 0
THP collapse fail 0 0
Compaction stalls 0 0
Compaction success 0 0
Compaction failures 0 0
Page migrate success 0 0
Page migrate failure 0 0
Compaction pages isolated 0 0
Compaction migrate scanned 0 0
Compaction free scanned 0 0
Compaction cost 0 0
NUMA PTE updates 0 0
NUMA hint faults 0 0
NUMA hint local faults 0 0
NUMA pages migrated 0 0
AutoNUMA cost 0 0
The zone allocation stats show that 26% of the allocations come out of
the Normal zone and 73% out of the DMA32 zone, which is equivalent to
their proportional share of physical memory.
---
micro
mmotm mmotm
unfair fairpcp
User 650.11 533.86
System 46.16 31.49
Elapsed 903.53 349.29
mmotm mmotm
unfair fairpcp
Page Ins 27582876 11116604
Page Outs 33959012 16573856
Swap Ins 0 0
Swap Outs 0 0
Alloc DMA 0 0
Alloc DMA32 8709355 6046277
Alloc Normal 3188567 1959526
Alloc Movable 0 0
Direct pages scanned 2588172 549598
Kswapd pages scanned 14803621 8451319
Kswapd pages reclaimed 6845369 3671141
Direct pages reclaimed 559581 229586
Kswapd efficiency 46% 43%
Kswapd velocity 16384.205 24195.708
Direct efficiency 21% 41%
Direct velocity 2864.511 1573.472
Percentage direct scans 14% 6%
Page writes by reclaim 0 0
Page writes file 0 0
Page writes anon 0 0
Page reclaim immediate 1 0
Page rescued immediate 0 0
Slabs scanned 9088 7936
Direct inode steals 3910 0
Kswapd inode steals 15793 14
Kswapd skipped wait 0 0
THP fault alloc 6350 5482
THP collapse alloc 1 0
THP splits 0 0
THP fault fallback 164 1622
THP collapse fail 0 0
Compaction stalls 126 324
Compaction success 40 57
Compaction failures 86 267
Page migrate success 16633 96349
Page migrate failure 0 0
Compaction pages isolated 49154 305649
Compaction migrate scanned 92448 414357
Compaction free scanned 598137 4971412
Compaction cost 18 108
NUMA PTE updates 0 0
NUMA hint faults 0 0
NUMA hint local faults 0 0
NUMA pages migrated 0 0
AutoNUMA cost 0 0
Elapsed time in comparison with user and sys time indicates much
reduced IO wait.
Interestingly, the allocations end up spreading out even in the
unpatched case, but only because kswapd seems to get stuck frequently
in a small Normal zone full of young dirty pages.
---
parallelio
mmotm mmotm
unfair fairpcp
Ops memcachetest-0M 28012.00 ( 0.00%) 27887.00 ( -0.45%)
Ops memcachetest-1877M 22366.00 ( 0.00%) 27878.00 ( 24.64%)
Ops memcachetest-6257M 17770.00 ( 0.00%) 27610.00 ( 55.37%)
Ops memcachetest-10638M 17695.00 ( 0.00%) 27350.00 ( 54.56%)
Ops io-duration-0M 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops io-duration-1877M 42.00 ( 0.00%) 18.00 ( 57.14%)
Ops io-duration-6257M 97.00 ( 0.00%) 57.00 ( 41.24%)
Ops io-duration-10638M 172.00 ( 0.00%) 122.00 ( 29.07%)
Ops swaptotal-0M 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops swaptotal-1877M 93603.00 ( 0.00%) 0.00 ( 0.00%)
Ops swaptotal-6257M 113986.00 ( 0.00%) 0.00 ( 0.00%)
Ops swaptotal-10638M 178887.00 ( 0.00%) 0.00 ( 0.00%)
Ops swapin-0M 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops swapin-1877M 20710.00 ( 0.00%) 0.00 ( 0.00%)
Ops swapin-6257M 18803.00 ( 0.00%) 0.00 ( 0.00%)
Ops swapin-10638M 18755.00 ( 0.00%) 0.00 ( 0.00%)
Ops minorfaults-0M 866454.00 ( 0.00%) 844880.00 ( 2.49%)
Ops minorfaults-1877M 957107.00 ( 0.00%) 845839.00 ( 11.63%)
Ops minorfaults-6257M 971144.00 ( 0.00%) 844778.00 ( 13.01%)
Ops minorfaults-10638M 1066811.00 ( 0.00%) 843628.00 ( 20.92%)
Ops majorfaults-0M 17.00 ( 0.00%) 0.00 ( 0.00%)
Ops majorfaults-1877M 7636.00 ( 0.00%) 37.00 ( 99.52%)
Ops majorfaults-6257M 6487.00 ( 0.00%) 37.00 ( 99.43%)
Ops majorfaults-10638M 7337.00 ( 0.00%) 37.00 ( 99.50%)
Mmtests reporting seems to have a bug calculating the percentage when
the numbers drop to 0, see swap activity. Those should all be 100%.
mmotm mmotm
unfair fairpcp
User 592.67 695.15
System 3130.44 3628.81
Elapsed 7209.01 7206.46
mmotm mmotm
unfair fairpcp
Page Ins 1401120 42656
Page Outs 163980516 153864256
Swap Ins 316033 0
Swap Outs 2528278 0
Alloc DMA 0 0
Alloc DMA32 59139091 51707843
Alloc Normal 10013244 16310697
Alloc Movable 0 0
Direct pages scanned 210080 235649
Kswapd pages scanned 61960450 50130023
Kswapd pages reclaimed 34998767 35769908
Direct pages reclaimed 179655 173478
Kswapd efficiency 56% 71%
Kswapd velocity 8594.863 6956.262
Direct efficiency 85% 73%
Direct velocity 29.141 32.700
Percentage direct scans 0% 0%
Page writes by reclaim 3523501 1
Page writes file 995223 1
Page writes anon 2528278 0
Page reclaim immediate 2195 9188
Page rescued immediate 0 0
Slabs scanned 2048 1536
Direct inode steals 0 0
Kswapd inode steals 0 0
Kswapd skipped wait 0 0
THP fault alloc 3 3
THP collapse alloc 4958 3026
THP splits 28 27
THP fault fallback 0 0
THP collapse fail 7 72
Compaction stalls 65 32
Compaction success 57 14
Compaction failures 8 18
Page migrate success 39460 9899
Page migrate failure 0 0
Compaction pages isolated 87140 22017
Compaction migrate scanned 53494 12526
Compaction free scanned 913691 396861
Compaction cost 42 10
NUMA PTE updates 0 0
NUMA hint faults 0 0
NUMA hint local faults 0 0
NUMA pages migrated 0 0
AutoNUMA cost 0 0
---
Patch on top of mmotm:
---
From: Johannes Weiner <hannes@xxxxxxxxxxx>
Subject: [patch] mm: page_alloc: use vmstats for fair zone allocation batching
Avoid dirtying the same cache line with every single page allocation
by making the fair per-zone allocation batch a vmstat item, which will
turn it into batched percpu counters on SMP.
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
include/linux/mmzone.h | 2 +-
mm/page_alloc.c | 21 ++++++++++++---------
mm/vmstat.c | 1 +
3 files changed, 14 insertions(+), 10 deletions(-)
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index dcad2ab..ac1ea79 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -105,6 +105,7 @@ struct zone_padding {
enum zone_stat_item {
/* First 128 byte cacheline (assuming 64 bit words) */
NR_FREE_PAGES,
+ NR_ALLOC_BATCH,
NR_LRU_BASE,
NR_INACTIVE_ANON = NR_LRU_BASE, /* must match order of LRU_[IN]ACTIVE */
NR_ACTIVE_ANON, /* " " " " " */
@@ -352,7 +353,6 @@ struct zone {
* free areas of different sizes
*/
spinlock_t lock;
- int alloc_batch;
int all_unreclaimable; /* All pages pinned */
#if defined CONFIG_COMPACTION || defined CONFIG_CMA
/* Set to true when the PG_migrate_skip bits should be cleared */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0d7e9e9..6a95d39 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1554,6 +1554,7 @@ again:
get_pageblock_migratetype(page));
}
+ __mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));
__count_zone_vm_events(PGALLOC, zone, 1 << order);
zone_statistics(preferred_zone, zone, gfp_flags);
local_irq_restore(flags);
@@ -1927,7 +1928,7 @@ zonelist_scan:
* fairness round-robin cycle of this zonelist.
*/
if (alloc_flags & ALLOC_WMARK_LOW) {
- if (zone->alloc_batch <= 0)
+ if (zone_page_state(zone, NR_ALLOC_BATCH) <= 0)
continue;
if (zone_reclaim_mode &&
!zone_local(preferred_zone, zone))
@@ -2039,8 +2040,7 @@ this_zone_full:
goto zonelist_scan;
}
- if (page) {
- zone->alloc_batch -= 1U << order;
+ if (page)
/*
* page->pfmemalloc is set when ALLOC_NO_WATERMARKS was
* necessary to allocate the page. The expectation is
@@ -2049,7 +2049,6 @@ this_zone_full:
* for !PFMEMALLOC purposes.
*/
page->pfmemalloc = !!(alloc_flags & ALLOC_NO_WATERMARKS);
- }
return page;
}
@@ -2418,8 +2417,10 @@ static void prepare_slowpath(gfp_t gfp_mask, unsigned int order,
*/
if (zone_reclaim_mode && !zone_local(preferred_zone, zone))
continue;
- zone->alloc_batch = high_wmark_pages(zone) -
- low_wmark_pages(zone);
+ mod_zone_page_state(zone, NR_ALLOC_BATCH,
+ high_wmark_pages(zone) -
+ low_wmark_pages(zone) -
+ zone_page_state(zone, NR_ALLOC_BATCH));
}
}
@@ -4827,7 +4828,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
zone->zone_pgdat = pgdat;
/* For bootup, initialized properly in watermark setup */
- zone->alloc_batch = zone->managed_pages;
+ mod_zone_page_state(zone, NR_ALLOC_BATCH, zone->managed_pages);
zone_pcp_init(zone);
lruvec_init(&zone->lruvec);
@@ -5606,8 +5607,10 @@ static void __setup_per_zone_wmarks(void)
zone->watermark[WMARK_LOW] = min_wmark_pages(zone) + (tmp >> 2);
zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + (tmp >> 1);
- zone->alloc_batch = high_wmark_pages(zone) -
- low_wmark_pages(zone);
+ __mod_zone_page_state(zone, NR_ALLOC_BATCH,
+ high_wmark_pages(zone) -
+ low_wmark_pages(zone) -
+ zone_page_state(zone, NR_ALLOC_BATCH));
setup_zone_migrate_reserve(zone);
spin_unlock_irqrestore(&zone->lock, flags);
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 87228c5..ba9e46b 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -704,6 +704,7 @@ static void walk_zones_in_node(struct seq_file *m, pg_data_t *pgdat,
const char * const vmstat_text[] = {
/* Zoned VM counters */
"nr_free_pages",
+ "nr_alloc_batch",
"nr_inactive_anon",
"nr_active_anon",
"nr_inactive_file",
--
1.8.3.2
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