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Date: Mon, 2 May 2011 18:29:45 +0800
From: Wu Fengguang <fengguang.wu@...el.com>
To: Minchan Kim <minchan.kim@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Mel Gorman <mel@...ux.vnet.ibm.com>,
Dave Young <hidave.darkstar@...il.com>,
linux-mm <linux-mm@...ck.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Christoph Lameter <cl@...ux.com>,
Dave Chinner <david@...morbit.com>,
David Rientjes <rientjes@...gle.com>,
Li Shaohua <shaohua.li@...el.com>,
Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC][PATCH] mm: cut down __GFP_NORETRY page allocation
failures
Hi Minchan,
On Mon, May 02, 2011 at 12:35:42AM +0800, Minchan Kim wrote:
> Hi Wu,
>
> On Sat, Apr 30, 2011 at 10:17:41PM +0800, Wu Fengguang wrote:
> > On Fri, Apr 29, 2011 at 10:28:24AM +0800, Wu Fengguang wrote:
> > > > Test results:
> > > >
> > > > - the failure rate is pretty sensible to the page reclaim size,
> > > > from 282 (WMARK_HIGH) to 704 (WMARK_MIN) to 10496 (SWAP_CLUSTER_MAX)
> > > >
> > > > - the IPIs are reduced by over 100 times
> > >
> > > It's reduced by 500 times indeed.
> > >
> > > CAL: 220449 220246 220372 220558 220251 219740 220043 219968 Function call interrupts
> > > CAL: 93 463 410 540 298 282 272 306 Function call interrupts
> > >
> > > > base kernel: vanilla 2.6.39-rc3 + __GFP_NORETRY readahead page allocation patch
> > > > -------------------------------------------------------------------------------
> > > > nr_alloc_fail 10496
> > > > allocstall 1576602
> > >
> > > > patched (WMARK_MIN)
> > > > -------------------
> > > > nr_alloc_fail 704
> > > > allocstall 105551
> > >
> > > > patched (WMARK_HIGH)
> > > > --------------------
> > > > nr_alloc_fail 282
> > > > allocstall 53860
> > >
> > > > this patch (WMARK_HIGH, limited scan)
> > > > -------------------------------------
> > > > nr_alloc_fail 276
> > > > allocstall 54034
> > >
> > > There is a bad side effect though: the much reduced "allocstall" means
> > > each direct reclaim will take much more time to complete. A simple solution
> > > is to terminate direct reclaim after 10ms. I noticed that an 100ms
> > > time threshold can reduce the reclaim latency from 621ms to 358ms.
> > > Further lowering the time threshold to 20ms does not help reducing the
> > > real latencies though.
> >
> > Experiments going on...
> >
> > I tried the more reasonable terminate condition: stop direct reclaim
> > when the preferred zone is above high watermark (see the below chunk).
> >
> > This helps reduce the average reclaim latency to under 100ms in the
> > 1000-dd case.
> >
> > However nr_alloc_fail is around 5000 and not ideal. The interesting
> > thing is, even if zone watermark is high, the task still may fail to
> > get a free page..
> >
> > @@ -2067,8 +2072,17 @@ static unsigned long do_try_to_free_page
> > }
> > }
> > total_scanned += sc->nr_scanned;
> > - if (sc->nr_reclaimed >= sc->nr_to_reclaim)
> > - goto out;
> > + if (sc->nr_reclaimed >= min_reclaim) {
> > + if (sc->nr_reclaimed >= sc->nr_to_reclaim)
> > + goto out;
> > + if (total_scanned > 2 * sc->nr_to_reclaim)
> > + goto out;
> > + if (preferred_zone &&
> > + zone_watermark_ok_safe(preferred_zone, sc->order,
> > + high_wmark_pages(preferred_zone),
> > + zone_idx(preferred_zone), 0))
> > + goto out;
> > + }
> >
> > /*
> > * Try to write back as many pages as we just scanned. This
> >
> > Thanks,
> > Fengguang
> > ---
> > Subject: mm: cut down __GFP_NORETRY page allocation failures
> > Date: Thu Apr 28 13:46:39 CST 2011
> >
> > Concurrent page allocations are suffering from high failure rates.
> >
> > On a 8p, 3GB ram test box, when reading 1000 sparse files of size 1GB,
> > the page allocation failures are
> >
> > nr_alloc_fail 733 # interleaved reads by 1 single task
> > nr_alloc_fail 11799 # concurrent reads by 1000 tasks
> >
> > The concurrent read test script is:
> >
> > for i in `seq 1000`
> > do
> > truncate -s 1G /fs/sparse-$i
> > dd if=/fs/sparse-$i of=/dev/null &
> > done
> >
> > In order for get_page_from_freelist() to get free page,
> >
> > (1) try_to_free_pages() should use much higher .nr_to_reclaim than the
> > current SWAP_CLUSTER_MAX=32, in order to draw the zone out of the
> > possible low watermark state as well as fill the pcp with enough free
> > pages to overflow its high watermark.
> >
> > (2) the get_page_from_freelist() _after_ direct reclaim should use lower
> > watermark than its normal invocations, so that it can reasonably
> > "reserve" some free pages for itself and prevent other concurrent
> > page allocators stealing all its reclaimed pages.
>
> Do you see my old patch? The patch want't incomplet but it's not bad for showing an idea.
> http://marc.info/?l=linux-mm&m=129187231129887&w=4
> The idea is to keep a page at leat for direct reclaimed process.
> Could it mitigate your problem or could you enhacne the idea?
> I think it's very simple and fair solution.
No it's not helping my problem, nr_alloc_fail and CAL are still high:
root@fat /home/wfg# ./test-dd-sparse.sh
start time: 246
total time: 531
nr_alloc_fail 14097
allocstall 1578332
LOC: 542698 538947 536986 567118 552114 539605 541201 537623 Local timer interrupts
RES: 3368 1908 1474 1476 2809 1602 1500 1509 Rescheduling interrupts
CAL: 223844 224198 224268 224436 223952 224056 223700 223743 Function call interrupts
TLB: 381 27 22 19 96 404 111 67 TLB shootdowns
root@fat /home/wfg# getdelays -dip `pidof dd`
print delayacct stats ON
printing IO accounting
PID 5202
CPU count real total virtual total delay total
1132 3635447328 3627947550 276722091605
IO count delay total delay average
2 187809974 62ms
SWAP count delay total delay average
0 0 0ms
RECLAIM count delay total delay average
1334 35304580824 26ms
dd: read=278528, write=0, cancelled_write=0
I guess your patch is mainly fixing the high order allocations while
my workload is mainly order 0 readahead page allocations. There are
1000 forks, however the "start time: 246" seems to indicate that the
order-1 reclaim latency is not improved.
I'll try modifying your patch and see how it works out. The obvious
change is to apply it to the order-0 case. Hope this won't create much
more isolated pages.
Attached is your patch rebased to 2.6.39-rc3, after resolving some
merge conflicts and fixing a trivial NULL pointer bug.
> >
> > Some notes:
> >
> > - commit 9ee493ce ("mm: page allocator: drain per-cpu lists after direct
> > reclaim allocation fails") has the same target, however is obviously
> > costly and less effective. It seems more clean to just remove the
> > retry and drain code than to retain it.
>
> Tend to agree.
> My old patch can solve it, I think.
Sadly nope. See above.
> >
> > - it's a bit hacky to reclaim more than requested pages inside
> > do_try_to_free_page(), and it won't help cgroup for now
> >
> > - it only aims to reduce failures when there are plenty of reclaimable
> > pages, so it stops the opportunistic reclaim when scanned 2 times pages
> >
> > Test results:
> >
> > - the failure rate is pretty sensible to the page reclaim size,
> > from 282 (WMARK_HIGH) to 704 (WMARK_MIN) to 10496 (SWAP_CLUSTER_MAX)
> >
> > - the IPIs are reduced by over 100 times
> >
> > base kernel: vanilla 2.6.39-rc3 + __GFP_NORETRY readahead page allocation patch
> > -------------------------------------------------------------------------------
> > nr_alloc_fail 10496
> > allocstall 1576602
> >
> > slabs_scanned 21632
> > kswapd_steal 4393382
> > kswapd_inodesteal 124
> > kswapd_low_wmark_hit_quickly 885
> > kswapd_high_wmark_hit_quickly 2321
> > kswapd_skip_congestion_wait 0
> > pageoutrun 29426
> >
> > CAL: 220449 220246 220372 220558 220251 219740 220043 219968 Function call interrupts
> >
> > LOC: 536274 532529 531734 536801 536510 533676 534853 532038 Local timer interrupts
> > RES: 3032 2128 1792 1765 2184 1703 1754 1865 Rescheduling interrupts
> > TLB: 189 15 13 17 64 294 97 63 TLB shootdowns
> >
> > patched (WMARK_MIN)
> > -------------------
> > nr_alloc_fail 704
> > allocstall 105551
> >
> > slabs_scanned 33280
> > kswapd_steal 4525537
> > kswapd_inodesteal 187
> > kswapd_low_wmark_hit_quickly 4980
> > kswapd_high_wmark_hit_quickly 2573
> > kswapd_skip_congestion_wait 0
> > pageoutrun 35429
> >
> > CAL: 93 286 396 754 272 297 275 281 Function call interrupts
> >
> > LOC: 520550 517751 517043 522016 520302 518479 519329 517179 Local timer interrupts
> > RES: 2131 1371 1376 1269 1390 1181 1409 1280 Rescheduling interrupts
> > TLB: 280 26 27 30 65 305 134 75 TLB shootdowns
> >
> > patched (WMARK_HIGH)
> > --------------------
> > nr_alloc_fail 282
> > allocstall 53860
> >
> > slabs_scanned 23936
> > kswapd_steal 4561178
> > kswapd_inodesteal 0
> > kswapd_low_wmark_hit_quickly 2760
> > kswapd_high_wmark_hit_quickly 1748
> > kswapd_skip_congestion_wait 0
> > pageoutrun 32639
> >
> > CAL: 93 463 410 540 298 282 272 306 Function call interrupts
> >
> > LOC: 513956 510749 509890 514897 514300 512392 512825 510574 Local timer interrupts
> > RES: 1174 2081 1411 1320 1742 2683 1380 1230 Rescheduling interrupts
> > TLB: 274 21 19 22 57 317 131 61 TLB shootdowns
> >
> > patched (WMARK_HIGH, limited scan)
> > ----------------------------------
> > nr_alloc_fail 276
> > allocstall 54034
> >
> > slabs_scanned 24320
> > kswapd_steal 4507482
> > kswapd_inodesteal 262
> > kswapd_low_wmark_hit_quickly 2638
> > kswapd_high_wmark_hit_quickly 1710
> > kswapd_skip_congestion_wait 0
> > pageoutrun 32182
> >
> > CAL: 69 443 421 567 273 279 269 334 Function call interrupts
>
> Looks amazing.
Yeah, I have strong feelings against drain_all_pages() in the direct
reclaim path. The intuition is, once drain_all_pages() is called, the
later on direct reclaims will have less chance to fill the drained
buffers and therefore forced into drain_all_pages() again and again.
drain_all_pages() is probably an overkill for preventing OOM.
Generally speaking, it's questionable to "squeeze the last page before
OOM".
A typical desktop enters thrashing storms before OOM, as Hugh pointed
out, this may well not the end users wanted. I agree with him and
personally prefer some applications to be OOM killed rather than the
whole system goes unusable thrashing like mad.
> > LOC: 514736 511698 510993 514069 514185 512986 513838 511229 Local timer interrupts
> > RES: 2153 1556 1126 1351 3047 1554 1131 1560 Rescheduling interrupts
> > TLB: 209 26 20 15 71 315 117 71 TLB shootdowns
> >
> > patched (WMARK_HIGH, limited scan, stop on watermark OK), 100 dd
> > ----------------------------------------------------------------
> >
> > start time: 3
> > total time: 50
> > nr_alloc_fail 162
> > allocstall 45523
> >
> > CPU count real total virtual total delay total
> > 921 3024540200 3009244668 37123129525
> > IO count delay total delay average
> > 0 0 0ms
> > SWAP count delay total delay average
> > 0 0 0ms
> > RECLAIM count delay total delay average
> > 357 4891766796 13ms
> > dd: read=0, write=0, cancelled_write=0
> >
> > patched (WMARK_HIGH, limited scan, stop on watermark OK), 1000 dd
> > -----------------------------------------------------------------
> >
> > start time: 272
> > total time: 509
> > nr_alloc_fail 3913
> > allocstall 541789
> >
> > CPU count real total virtual total delay total
> > 1044 3445476208 3437200482 229919915202
> > IO count delay total delay average
> > 0 0 0ms
> > SWAP count delay total delay average
> > 0 0 0ms
> > RECLAIM count delay total delay average
> > 452 34691441605 76ms
> > dd: read=0, write=0, cancelled_write=0
> >
> > patched (WMARK_HIGH, limited scan, stop on watermark OK, no time limit), 1000 dd
> > --------------------------------------------------------------------------------
> >
> > start time: 278
> > total time: 513
> > nr_alloc_fail 4737
> > allocstall 436392
> >
> >
> > CPU count real total virtual total delay total
> > 1024 3371487456 3359441487 225088210977
> > IO count delay total delay average
> > 1 160631171 160ms
> > SWAP count delay total delay average
> > 0 0 0ms
> > RECLAIM count delay total delay average
> > 367 30809994722 83ms
> > dd: read=20480, write=0, cancelled_write=0
> >
> >
> > no cond_resched():
>
> What's this?
I tried a modified patch that also removes the cond_resched() call in
__alloc_pages_direct_reclaim(), between try_to_free_pages() and
get_page_from_freelist(). It seems not helping noticeably.
It looks safe to remove that cond_resched() as we already have such
calls in shrink_page_list().
> >
> > start time: 263
> > total time: 516
> > nr_alloc_fail 5144
> > allocstall 436787
> >
> > CPU count real total virtual total delay total
> > 1018 3305497488 3283831119 241982934044
> > IO count delay total delay average
> > 0 0 0ms
> > SWAP count delay total delay average
> > 0 0 0ms
> > RECLAIM count delay total delay average
> > 328 31398481378 95ms
> > dd: read=0, write=0, cancelled_write=0
> >
> > zone_watermark_ok_safe():
> >
> > start time: 266
> > total time: 513
> > nr_alloc_fail 4526
> > allocstall 440246
> >
> > CPU count real total virtual total delay total
> > 1119 3640446568 3619184439 240945024724
> > IO count delay total delay average
> > 3 303620082 101ms
> > SWAP count delay total delay average
> > 0 0 0ms
> > RECLAIM count delay total delay average
> > 372 27320731898 73ms
> > dd: read=77824, write=0, cancelled_write=0
> >
> > start time: 275
>
> What's meaing of start time?
It's the time taken to start 1000 dd's.
> > total time: 517
>
> Total time is elapsed time on your experiment?
Yeah. They are generated with this script.
$ cat ~/bin/test-dd-sparse.sh
#!/bin/sh
mount /dev/sda7 /fs
tic=$(date +'%s')
for i in `seq 1000`
do
truncate -s 1G /fs/sparse-$i
dd if=/fs/sparse-$i of=/dev/null &>/dev/null &
done
tac=$(date +'%s')
echo start time: $((tac-tic))
wait
tac=$(date +'%s')
echo total time: $((tac-tic))
egrep '(nr_alloc_fail|allocstall)' /proc/vmstat
egrep '(CAL|RES|LOC|TLB)' /proc/interrupts
> > nr_alloc_fail 4694
> > allocstall 431021
> >
> >
> > CPU count real total virtual total delay total
> > 1073 3534462680 3512544928 234056498221
>
> What's meaning of CPU fields?
It's "waiting for a CPU (while being runnable)" as described in
Documentation/accounting/delay-accounting.txt.
> > IO count delay total delay average
> > 0 0 0ms
> > SWAP count delay total delay average
> > 0 0 0ms
> > RECLAIM count delay total delay average
> > 386 34751778363 89ms
> > dd: read=0, write=0, cancelled_write=0
> >
>
> Where is vanilla data for comparing latency?
> Personally, It's hard to parse your data.
Sorry it's somehow too much data and kernel revisions.. The base kernel's
average latency is 29ms:
base kernel: vanilla 2.6.39-rc3 + __GFP_NORETRY readahead page allocation patch
-------------------------------------------------------------------------------
CPU count real total virtual total delay total
1122 3676441096 3656793547 274182127286
IO count delay total delay average
3 291765493 97ms
SWAP count delay total delay average
0 0 0ms
RECLAIM count delay total delay average
1350 39229752193 29ms
dd: read=45056, write=0, cancelled_write=0
start time: 245
total time: 526
nr_alloc_fail 14586
allocstall 1578343
LOC: 533981 529210 528283 532346 533392 531314 531705 528983 Local timer interrupts
RES: 3123 2177 1676 1580 2157 1974 1606 1696 Rescheduling interrupts
CAL: 218392 218631 219167 219217 218840 218985 218429 218440 Function call interrupts
TLB: 175 13 21 18 62 309 119 42 TLB shootdowns
>
> > CC: Mel Gorman <mel@...ux.vnet.ibm.com>
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > ---
> > fs/buffer.c | 4 ++--
> > include/linux/swap.h | 3 ++-
> > mm/page_alloc.c | 20 +++++---------------
> > mm/vmscan.c | 31 +++++++++++++++++++++++--------
> > 4 files changed, 32 insertions(+), 26 deletions(-)
> > --- linux-next.orig/mm/vmscan.c 2011-04-29 10:42:14.000000000 +0800
> > +++ linux-next/mm/vmscan.c 2011-04-30 21:59:33.000000000 +0800
> > @@ -2025,8 +2025,9 @@ static bool all_unreclaimable(struct zon
> > * returns: 0, if no pages reclaimed
> > * else, the number of pages reclaimed
> > */
> > -static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
> > - struct scan_control *sc)
> > +static unsigned long do_try_to_free_pages(struct zone *preferred_zone,
> > + struct zonelist *zonelist,
> > + struct scan_control *sc)
> > {
> > int priority;
> > unsigned long total_scanned = 0;
> > @@ -2034,6 +2035,7 @@ static unsigned long do_try_to_free_page
> > struct zoneref *z;
> > struct zone *zone;
> > unsigned long writeback_threshold;
> > + unsigned long min_reclaim = sc->nr_to_reclaim;
>
> Hmm,
>
> >
> > get_mems_allowed();
> > delayacct_freepages_start();
> > @@ -2041,6 +2043,9 @@ static unsigned long do_try_to_free_page
> > if (scanning_global_lru(sc))
> > count_vm_event(ALLOCSTALL);
> >
> > + if (preferred_zone)
> > + sc->nr_to_reclaim += preferred_zone->watermark[WMARK_HIGH];
> > +
>
> Hmm, I don't like this idea.
> The goal of direct reclaim path is to reclaim pages asap, I beleive.
> Many thing should be achieve of background kswapd.
> If admin changes min_free_kbytes, it can affect latency of direct reclaim.
> It doesn't make sense to me.
Yeah, it does increase delays.. in the 1000 dd case, roughly from 30ms
to 90ms. This is a major drawback.
> > for (priority = DEF_PRIORITY; priority >= 0; priority--) {
> > sc->nr_scanned = 0;
> > if (!priority)
> > @@ -2067,8 +2072,17 @@ static unsigned long do_try_to_free_page
> > }
> > }
> > total_scanned += sc->nr_scanned;
> > - if (sc->nr_reclaimed >= sc->nr_to_reclaim)
> > - goto out;
> > + if (sc->nr_reclaimed >= min_reclaim) {
> > + if (sc->nr_reclaimed >= sc->nr_to_reclaim)
> > + goto out;
>
> I can't understand the logic.
> if nr_reclaimed is bigger than min_reclaim, it's always greater than
> nr_to_reclaim. What's meaning of min_reclaim?
In direct reclaim, min_reclaim will be the legacy SWAP_CLUSTER_MAX and
sc->nr_to_reclaim will be increased to the zone's high watermark and
is kind of "max to reclaim".
>
> > + if (total_scanned > 2 * sc->nr_to_reclaim)
> > + goto out;
>
> If there are lots of dirty pages in LRU?
> If there are lots of unevictable pages in LRU?
> If there are lots of mapped page in LRU but may_unmap = 0 cases?
> I means it's rather risky early conclusion.
That test means to avoid scanning too much on __GFP_NORETRY direct
reclaims. My assumption for __GFP_NORETRY is, it should fail fast when
the LRU pages seem hard to reclaim. And the problem in the 1000 dd
case is, it's all easy to reclaim LRU pages but __GFP_NORETRY still
fails from time to time, with lots of IPIs that may hurt large
machines a lot.
>
> > + if (preferred_zone &&
> > + zone_watermark_ok_safe(preferred_zone, sc->order,
> > + high_wmark_pages(preferred_zone),
> > + zone_idx(preferred_zone), 0))
> > + goto out;
> > + }
>
> As I said, I think direct reclaim path sould be fast if possbile and
> it should not a function of min_free_kbytes.
Right.
> Of course, there are lots of tackle for keep direct reclaim path's consistent
> latency but at least, I don't want to add another source.
OK.
Thanks,
Fengguang
View attachment "mm-keep-freed-pages-in-direct-reclaim.patch" of type "text/x-diff" (6105 bytes)
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