Hi Satoru,
On Tue, May 03, 2011 at 08:27:43AM +0800, Satoru Moriya wrote:
> Hi Wu,
>
> > On Mon, May 02, 2011 at 09:29:58PM +0800, Wu Fengguang wrote:
> > > > > + 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.
> > >
> > > It can be made not a function of min_free_kbytes by simply changing
> > > high_wmark_pages() to low_wmark_pages() in the above chunk, since
> > > direct reclaim is triggered when ALLOC_WMARK_LOW cannot be satisfied,
> > > ie. it just dropped below low_wmark_pages().
> > >
> > > But still, it costs 62ms reclaim latency (base kernel is 29ms).
> >
> > I got new findings: the CPU schedule delays are much larger than
> > reclaim delays. It does make the "direct reclaim until low watermark
> > OK" latency less a problem :)
> >
> > 1000 dd test case:
> > RECLAIM delay CPU delay nr_alloc_fail CAL (last CPU)
> > base kernel 29ms 244ms 14586 218440
> > patched 62ms 215ms 5004 325
>
> Hmm, in your system, the latency of direct reclaim may be a less problem.
>
> But, generally speaking, in a latency sensitive system in enterprise area
> there are two kind of processes. One is latency sensitive -(A) the other
> is not-latency sensitive -(B). And usually we set cpu affinity for both processes
> to avoid scheduling issue in (A). In this situation, CPU delay tends to be lower
> than the above and a less problem but reclaim delay is more critical.
Good point, thanks!
I also tried increasing min_free_kbytes as indicated by Minchan and
find 1-second long reclaim delays... Even adding explicit time limits,
it's still over 100ms with very high nr_alloc_fail.
I'm listing the code and results here as a record. But in general I'll
stop experiments in this direction. We need some more oriented way
that can guarantee to satisfy the page allocation request after small
sized direct reclaims.
Thanks,
Fengguang
---
root@fat /home/wfg# ./test-dd-sparse.sh
start time: 250
total time: 518
nr_alloc_fail 18551
allocstall 234468
LOC: 525770 523124 520782 529151 526192 525004 524166 521527 Local timer interrupts
RES: 2174 1674 1301 1420 3329 1563 1314 1563 Rescheduling interrupts
CAL: 67 402 602 267 240 270 291 274 Function call interrupts
TLB: 197 25 23 17 80 321 121 58 TLB shootdowns
CPU count real total virtual total delay total delay average
1078 3408481832 3400786094 256971188317 238.378ms
IO count delay total delay average
5 414363739 82ms
SWAP count delay total delay average
0 0 0ms
RECLAIM count delay total delay average
187 28564728545 152ms
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
(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.
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.
- 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 (1000 dd case):
- the failure rate is pretty sensible to the page reclaim size,
from 282 (WMARK_HIGH) to 704 (WMARK_MIN) to 5004 (WMARK_HIGH, stop on low
watermark ok) to 10496 (SWAP_CLUSTER_MAX)
- the IPIs are reduced by over 500 times
- the reclaim delay is doubled, from 29ms to 62ms
Base kernel is vanilla 2.6.39-rc3 + __GFP_NORETRY readahead page allocations.
base kernel, 1000 dd
--------------------
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
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
patched, 1000 dd
----------------
root@fat /home/wfg# ./test-dd-sparse.sh
start time: 260
total time: 519
nr_alloc_fail 5004
allocstall 551429
LOC: 524861 521832 520945 524632 524666 523334 523797 521562 Local timer interrupts
RES: 1323 1976 2505 1610 1544 1848 3310 1644 Rescheduling interrupts
CAL: 67 335 353 614 289 287 293 325 Function call interrupts
TLB: 288 29 26 34 103 321 123 70 TLB shootdowns
CPU count real total virtual total delay total
1177 3797422704 3775174301 253228435955
IO count delay total delay average
1 198528820 198ms
SWAP count delay total delay average
0 0 0ms
RECLAIM count delay total delay average
508 31660219699 62ms
base kernel, 100 dd
-------------------
root@fat /home/wfg# ./test-dd-sparse.sh
start time: 3
total time: 53
nr_alloc_fail 849
allocstall 131330
LOC: 59843 56506 55838 65283 61774 57929 58880 56246 Local timer interrupts
RES: 376 308 372 239 374 307 491 239 Rescheduling interrupts
CAL: 17737 18083 17948 18192 17929 17845 17893 17906 Function call interrupts
TLB: 307 26 25 21 80 324 137 79 TLB shootdowns
CPU count real total virtual total delay total
974 3197513904 3180727460 38504429363
IO count delay total delay average
1 18156696 18ms
SWAP count delay total delay average
0 0 0ms
RECLAIM count delay total delay average
1036 3439387298 3ms
dd: read=12288, write=0, cancelled_write=0
patched, 100 dd
---------------
root@fat /home/wfg# ./test-dd-sparse.sh
start time: 3
total time: 52
nr_alloc_fail 307
allocstall 48178
LOC: 56486 53514 52792 55879 56317 55383 55311 53168 Local timer interrupts
RES: 604 345 257 250 775 371 272 252 Rescheduling interrupts
CAL: 75 373 369 543 272 278 295 296 Function call interrupts
TLB: 259 24 19 24 82 306 139 53 TLB shootdowns
CPU count real total virtual total delay total
974 3177516944 3161771347 38508053977
IO count delay total delay average
0 0 0ms
SWAP count delay total delay average
0 0 0ms
RECLAIM count delay total delay average
393 5389030889 13ms
dd: read=0, write=0, cancelled_write=0
CC: Mel Gorman <mel@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
fs/buffer.c | 4 ++--
include/linux/swap.h | 3 ++-
mm/page_alloc.c | 22 +++++-----------------
mm/vmscan.c | 34 ++++++++++++++++++++++++++--------
4 files changed, 35 insertions(+), 28 deletions(-)
--- linux-next.orig/mm/vmscan.c 2011-05-02 22:14:14.000000000 +0800
+++ linux-next/mm/vmscan.c 2011-05-03 10:07:14.000000000 +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 +2035,8 @@ 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;
+ unsigned long start_time = jiffies;
get_mems_allowed();
delayacct_freepages_start();
@@ -2041,6 +2044,9 @@ 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_LOW];
+
for (priority = DEF_PRIORITY; priority >= 0; priority--) {
sc->nr_scanned = 0;
if (!priority)
@@ -2067,8 +2073,19 @@ 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(preferred_zone, sc->order,
+ low_wmark_pages(preferred_zone),
+ zone_idx(preferred_zone), 0))
+ goto out;
+ if (jiffies - start_time > HZ / 100)
+ goto out;
+ }
/*
* Try to write back as many pages as we just scanned. This
@@ -2117,7 +2134,8 @@ out:
return 0;
}
-unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
+unsigned long try_to_free_pages(struct zone *preferred_zone,
+ struct zonelist *zonelist, int order,
gfp_t gfp_mask, nodemask_t *nodemask)
{
unsigned long nr_reclaimed;
@@ -2137,7 +2155,7 @@ unsigned long try_to_free_pages(struct z
sc.may_writepage,
gfp_mask);
- nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
+ nr_reclaimed = do_try_to_free_pages(preferred_zone, zonelist, &sc);
trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);
@@ -2207,7 +2225,7 @@ unsigned long try_to_free_mem_cgroup_pag
sc.may_writepage,
sc.gfp_mask);
- nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
+ nr_reclaimed = do_try_to_free_pages(NULL, zonelist, &sc);
trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
@@ -2796,7 +2814,7 @@ unsigned long shrink_all_memory(unsigned
reclaim_state.reclaimed_slab = 0;
p->reclaim_state = &reclaim_state;
- nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
+ nr_reclaimed = do_try_to_free_pages(NULL, zonelist, &sc);
p->reclaim_state = NULL;
lockdep_clear_current_reclaim_state();
--- linux-next.orig/mm/page_alloc.c 2011-05-02 22:14:14.000000000 +0800
+++ linux-next/mm/page_alloc.c 2011-05-02 22:14:21.000000000 +0800
@@ -1888,9 +1888,8 @@ __alloc_pages_direct_reclaim(gfp_t gfp_m
nodemask_t *nodemask, int alloc_flags, struct zone *preferred_zone,
int migratetype, unsigned long *did_some_progress)
{
- struct page *page = NULL;
+ struct page *page;
struct reclaim_state reclaim_state;
- bool drained = false;
cond_resched();
@@ -1901,33 +1900,22 @@ __alloc_pages_direct_reclaim(gfp_t gfp_m
reclaim_state.reclaimed_slab = 0;
current->reclaim_state = &reclaim_state;
- *did_some_progress = try_to_free_pages(zonelist, order, gfp_mask, nodemask);
+ *did_some_progress = try_to_free_pages(preferred_zone, zonelist, order,
+ gfp_mask, nodemask);
current->reclaim_state = NULL;
lockdep_clear_current_reclaim_state();
current->flags &= ~PF_MEMALLOC;
- cond_resched();
-
if (unlikely(!(*did_some_progress)))
return NULL;
-retry:
+ alloc_flags |= ALLOC_HARDER;
+
page = get_page_from_freelist(gfp_mask, nodemask, order,
zonelist, high_zoneidx,
alloc_flags, preferred_zone,
migratetype);
-
- /*
- * If an allocation failed after direct reclaim, it could be because
- * pages are pinned on the per-cpu lists. Drain them and try again
- */
- if (!page && !drained) {
- drain_all_pages();
- drained = true;
- goto retry;
- }
-
return page;
}
--- linux-next.orig/fs/buffer.c 2011-05-02 22:14:14.000000000 +0800
+++ linux-next/fs/buffer.c 2011-05-02 22:14:21.000000000 +0800
@@ -288,8 +288,8 @@ static void free_more_memory(void)
gfp_zone(GFP_NOFS), NULL,
&zone);
if (zone)
- try_to_free_pages(node_zonelist(nid, GFP_NOFS), 0,
- GFP_NOFS, NULL);
+ try_to_free_pages(zone, node_zonelist(nid, GFP_NOFS),
+ 0, GFP_NOFS, NULL);
}
}
--- linux-next.orig/include/linux/swap.h 2011-05-02 22:14:14.000000000 +0800
+++ linux-next/include/linux/swap.h 2011-05-02 22:14:21.000000000 +0800
@@ -249,7 +249,8 @@ static inline void lru_cache_add_file(st
#define ISOLATE_BOTH 2 /* Isolate both active and inactive pages. */
/* linux/mm/vmscan.c */
-extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
+extern unsigned long try_to_free_pages(struct zone *preferred_zone,
+ struct zonelist *zonelist, int order,
gfp_t gfp_mask, nodemask_t *mask);
extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem,
gfp_t gfp_mask, bool noswap,
--
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