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Date: Wed, 7 Aug 2013 11:37:43 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Mel Gorman <mgorman@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Rik van Riel <riel@...riel.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Zlatko Calusic <zcalusic@...sync.net>,
Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [patch v2 3/3] mm: page_alloc: fair zone allocator policy
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:
> > Each zone that holds userspace pages of one workload must be aged at a
> > speed proportional to the zone size. Otherwise, the time an
> > individual page gets to stay in memory depends on the zone it happened
> > to be allocated in. Asymmetry in the zone aging creates rather
> > unpredictable aging behavior and results in the wrong pages being
> > reclaimed, activated etc.
> >
> > But exactly this happens right now because of the way the page
> > allocator and kswapd interact. The page allocator uses per-node lists
> > of all zones in the system, ordered by preference, when allocating a
> > new page. When the first iteration does not yield any results, kswapd
> > is woken up and the allocator retries. Due to the way kswapd reclaims
> > zones below the high watermark while a zone can be allocated from when
> > it is above the low watermark, the allocator may keep kswapd running
> > while kswapd reclaim ensures that the page allocator can keep
> > allocating from the first zone in the zonelist for extended periods of
> > time. Meanwhile the other zones rarely see new allocations and thus
> > get aged much slower in comparison.
> >
> > The result is that the occasional page placed in lower zones gets
> > relatively more time in memory, even gets promoted to the active list
> > after its peers have long been evicted. Meanwhile, the bulk of the
> > working set may be thrashing on the preferred zone even though there
> > may be significant amounts of memory available in the lower zones.
> >
> > Even the most basic test -- repeatedly reading a file slightly bigger
> > than memory -- shows how broken the zone aging is. In this scenario,
> > no single page should be able stay in memory long enough to get
> > referenced twice and activated, but activation happens in spades:
> >
> > $ grep active_file /proc/zoneinfo
> > nr_inactive_file 0
> > nr_active_file 0
> > nr_inactive_file 0
> > nr_active_file 8
> > nr_inactive_file 1582
> > nr_active_file 11994
> > $ cat data data data data >/dev/null
> > $ grep active_file /proc/zoneinfo
> > nr_inactive_file 0
> > nr_active_file 70
> > nr_inactive_file 258753
> > nr_active_file 443214
> > nr_inactive_file 149793
> > nr_active_file 12021
> >
> > Fix this with a very simple round robin allocator. Each zone is
> > allowed a batch of allocations that is proportional to the zone's
> > size, after which it is treated as full. The batch counters are reset
> > when all zones have been tried and the allocator enters the slowpath
> > and kicks off kswapd reclaim. Allocation and reclaim is now fairly
> > spread out to all available/allowable zones:
> >
> > $ grep active_file /proc/zoneinfo
> > nr_inactive_file 0
> > nr_active_file 0
> > nr_inactive_file 174
> > nr_active_file 4865
> > nr_inactive_file 53
> > nr_active_file 860
> > $ cat data data data data >/dev/null
> > $ grep active_file /proc/zoneinfo
> > nr_inactive_file 0
> > nr_active_file 0
> > nr_inactive_file 666622
> > nr_active_file 4988
> > nr_inactive_file 190969
> > nr_active_file 937
> >
> > When zone_reclaim_mode is enabled, allocations will now spread out to
> > all zones on the local node, not just the first preferred zone (which
> > on a 4G node might be a tiny Normal zone).
> >
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > Tested-by: Zlatko Calusic <zcalusic@...sync.net>
> > ---
> > include/linux/mmzone.h | 1 +
> > mm/page_alloc.c | 69 ++++++++++++++++++++++++++++++++++++++++++--------
> > 2 files changed, 60 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index af4a3b7..dcad2ab 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -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.
> > @@ -2006,7 +2036,8 @@ this_zone_full:
> > goto zonelist_scan;
> > }
> >
> > - if (page)
> > + if (page) {
> > + zone->alloc_batch -= 1U << order;
>
> This line is where I think there will be noticable increases in cache
> misses when running parallel tests. PFT from mmtests on a large machine
> might be able to show the problem.
I tested this back then with the original atomic ops on a two socket
machine:
pft
BASE RRALLOC WORKINGSET
User 1 0.0235 ( 0.00%) 0.0275 (-17.02%) 0.0270 (-14.89%)
User 2 0.0275 ( 0.00%) 0.0275 ( -0.00%) 0.0285 ( -3.64%)
User 3 0.0330 ( 0.00%) 0.0365 (-10.61%) 0.0335 ( -1.52%)
User 4 0.0390 ( 0.00%) 0.0390 ( 0.00%) 0.0380 ( 2.56%)
System 1 0.2645 ( 0.00%) 0.2620 ( 0.95%) 0.2625 ( 0.76%)
System 2 0.3215 ( 0.00%) 0.3310 ( -2.95%) 0.3285 ( -2.18%)
System 3 0.3935 ( 0.00%) 0.4080 ( -3.68%) 0.4130 ( -4.96%)
System 4 0.4920 ( 0.00%) 0.5030 ( -2.24%) 0.5045 ( -2.54%)
Elapsed 1 0.2905 ( 0.00%) 0.2905 ( 0.00%) 0.2905 ( 0.00%)
Elapsed 2 0.1800 ( 0.00%) 0.1800 ( 0.00%) 0.1800 ( 0.00%)
Elapsed 3 0.1500 ( 0.00%) 0.1600 ( -6.67%) 0.1600 ( -6.67%)
Elapsed 4 0.1305 ( 0.00%) 0.1420 ( -8.81%) 0.1415 ( -8.43%)
Faults/cpu 1 667251.7997 ( 0.00%) 666296.4749 ( -0.14%) 667880.8099 ( 0.09%)
Faults/cpu 2 551464.0345 ( 0.00%) 536113.4630 ( -2.78%) 538286.2087 ( -2.39%)
Faults/cpu 3 452403.4425 ( 0.00%) 433856.5320 ( -4.10%) 432193.9888 ( -4.47%)
Faults/cpu 4 362691.4491 ( 0.00%) 356514.8821 ( -1.70%) 356436.5711 ( -1.72%)
Faults/sec 1 663612.5980 ( 0.00%) 662501.4959 ( -0.17%) 664037.3123 ( 0.06%)
Faults/sec 2 1096166.5317 ( 0.00%) 1064679.7154 ( -2.87%) 1068906.1040 ( -2.49%)
Faults/sec 3 1272925.4995 ( 0.00%) 1209241.9167 ( -5.00%) 1202868.9190 ( -5.50%)
Faults/sec 4 1437691.1054 ( 0.00%) 1362549.9877 ( -5.23%) 1381633.9889 ( -3.90%)
so a 2-5% regression in fault throughput on this machine. I would
love to avoid it, but I don't think it's a show stopper if it buys
500% improvements in tests like parallelio on the same machine.
> > @@ -2346,16 +2378,28 @@ __alloc_pages_high_priority(gfp_t gfp_mask, unsigned int order,
> > return page;
> > }
> >
> > -static inline
> > -void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
> > - enum zone_type high_zoneidx,
> > - enum zone_type classzone_idx)
> > +static void prepare_slowpath(gfp_t gfp_mask, unsigned int order,
> > + struct zonelist *zonelist,
> > + enum zone_type high_zoneidx,
> > + struct zone *preferred_zone)
> > {
> > struct zoneref *z;
> > struct zone *zone;
> >
> > - for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
> > - wakeup_kswapd(zone, order, classzone_idx);
> > + for_each_zone_zonelist(zone, z, zonelist, high_zoneidx) {
> > + if (!(gfp_mask & __GFP_NO_KSWAPD))
> > + wakeup_kswapd(zone, order, zone_idx(preferred_zone));
> > + /*
> > + * Only reset the batches of zones that were actually
> > + * considered in the fast path, we don't want to
> > + * thrash fairness information for zones that are not
> > + * actually part of this zonelist's round-robin cycle.
> > + */
> > + if (zone_reclaim_mode && !zone_local(preferred_zone, zone))
> > + continue;
> > + zone->alloc_batch = high_wmark_pages(zone) -
> > + low_wmark_pages(zone);
> > + }
>
> We now call wakeup_kswapd() when the batches for the round-robin are
> expired. In some circumstances this can be expensive in its own right if
> it calls zone_watermark_ok_safe() from zone_balanced().
>
> If we are entering the slowpath because the batches are expired should
> the fast path reset the alloc_batches once and retry the fast path before
> wakeup_kswapd?
The batches are set up so that their expiration coincides with the
watermarks being hit. I haven't actually double checked it, but I'm
going to run some tests to see if the wakeups increased significantly.
Thanks for the input, Mel!
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