Messages in this thread | | | Date | Thu, 20 Aug 2009 21:13:59 +0900 | Subject | Re: [PATCH -v2] mm: do batched scans for mem_cgroup | From | Minchan Kim <> |
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On Thu, Aug 20, 2009 at 8:49 PM, Wu Fengguang<fengguang.wu@intel.com> wrote: > On Thu, Aug 20, 2009 at 07:01:21PM +0800, Minchan Kim wrote: >> Hi, Wu. >> >> On Thu, Aug 20, 2009 at 1:05 PM, Wu Fengguang<fengguang.wu@intel.com> wrote: >> > On Thu, Aug 20, 2009 at 11:13:47AM +0800, KAMEZAWA Hiroyuki wrote: >> >> On Thu, 20 Aug 2009 10:49:29 +0800 >> >> Wu Fengguang <fengguang.wu@intel.com> wrote: >> >> >> >> > For mem_cgroup, shrink_zone() may call shrink_list() with nr_to_scan=1, >> >> > in which case shrink_list() _still_ calls isolate_pages() with the much >> >> > larger SWAP_CLUSTER_MAX. It effectively scales up the inactive list >> >> > scan rate by up to 32 times. >> >> > >> >> > For example, with 16k inactive pages and DEF_PRIORITY=12, (16k >> 12)=4. >> >> > So when shrink_zone() expects to scan 4 pages in the active/inactive >> >> > list, it will be scanned SWAP_CLUSTER_MAX=32 pages in effect. >> >> > >> >> > The accesses to nr_saved_scan are not lock protected and so not 100% >> >> > accurate, however we can tolerate small errors and the resulted small >> >> > imbalanced scan rates between zones. >> >> > >> >> > This batching won't blur up the cgroup limits, since it is driven by >> >> > "pages reclaimed" rather than "pages scanned". When shrink_zone() >> >> > decides to cancel (and save) one smallish scan, it may well be called >> >> > again to accumulate up nr_saved_scan. >> >> > >> >> > It could possibly be a problem for some tiny mem_cgroup (which may be >> >> > _full_ scanned too much times in order to accumulate up nr_saved_scan). >> >> > >> >> > CC: Rik van Riel <riel@redhat.com> >> >> > CC: Minchan Kim <minchan.kim@gmail.com> >> >> > CC: Balbir Singh <balbir@linux.vnet.ibm.com> >> >> > CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> >> >> > CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> >> >> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> >> >> > --- >> >> >> >> Hmm, how about this ? >> >> == >> >> Now, nr_saved_scan is tied to zone's LRU. >> >> But, considering how vmscan works, it should be tied to reclaim_stat. >> >> >> >> By this, memcg can make use of nr_saved_scan information seamlessly. >> > >> > Good idea, full patch updated with your signed-off-by :) >> > >> > Thanks, >> > Fengguang >> > --- >> > mm: do batched scans for mem_cgroup >> > >> > For mem_cgroup, shrink_zone() may call shrink_list() with nr_to_scan=1, >> > in which case shrink_list() _still_ calls isolate_pages() with the much >> > larger SWAP_CLUSTER_MAX. It effectively scales up the inactive list >> > scan rate by up to 32 times. >> >> Yes. It can scan 32 times pages in only inactive list, not active list. > > Yes and no ;) > > inactive anon list over scanned => inactive_anon_is_low() == TRUE > => shrink_active_list() > => active anon list over scanned
Why inactive anon list is overscanned in case mem_cgroup ?
in shrink_zone, 1) The vm doesn't accumulate nr[l]. 2) Below routine store min value to nr_to_scan. nr_to_scan = min(nr[l], swap_cluster_max); ex) if nr[l] = 4, vm calls shrink_active_list with 4 as nr_to_scan. So I think overscan doesn't occur in active list.
> So the end result may be > > - anon inactive => over scanned > - anon active => over scanned (maybe not as much) > - file inactive => over scanned > - file active => under scanned (relatively) > >> > For example, with 16k inactive pages and DEF_PRIORITY=12, (16k >> 12)=4. >> > So when shrink_zone() expects to scan 4 pages in the active/inactive >> > list, it will be scanned SWAP_CLUSTER_MAX=32 pages in effect. >> >> Active list scan would be scanned in 4, inactive list is 32. > > Exactly. > >> > >> > The accesses to nr_saved_scan are not lock protected and so not 100% >> > accurate, however we can tolerate small errors and the resulted small >> > imbalanced scan rates between zones. >> >> Yes. >> >> > This batching won't blur up the cgroup limits, since it is driven by >> > "pages reclaimed" rather than "pages scanned". When shrink_zone() >> > decides to cancel (and save) one smallish scan, it may well be called >> > again to accumulate up nr_saved_scan. >> >> You mean nr_scan_try_batch logic ? >> But that logic works for just global reclaim? >> Now am I missing something? >> >> Could you elaborate more? :) > > Sorry for the confusion. The above paragraph originates from Balbir's > concern: > > This might be a concern (although not a big ATM), since we can't > afford to miss limits by much. If a cgroup is near its limit and we > drop scanning it. We'll have to work out what this means for the end
Why does mem_cgroup drops scanning ? It's because nr_scan_try_batch? or something ?
Sorry. Still, I can't understand your point. :(
> user. May be more fundamental look through is required at the priority > based logic of exposing how much to scan, I don't know. > > Thanks, > Fengguang > >> > It could possibly be a problem for some tiny mem_cgroup (which may be >> > _full_ scanned too much times in order to accumulate up nr_saved_scan). >> > >> > CC: Rik van Riel <riel@redhat.com> >> > CC: Minchan Kim <minchan.kim@gmail.com> >> > CC: Balbir Singh <balbir@linux.vnet.ibm.com> >> > CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> >> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> >> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> >> > --- >> > include/linux/mmzone.h | 6 +++++- >> > mm/page_alloc.c | 2 +- >> > mm/vmscan.c | 20 +++++++++++--------- >> > 3 files changed, 17 insertions(+), 11 deletions(-) >> > >> > --- linux.orig/include/linux/mmzone.h 2009-07-30 10:45:15.000000000 +0800 >> > +++ linux/include/linux/mmzone.h 2009-08-20 11:51:08.000000000 +0800 >> > @@ -269,6 +269,11 @@ struct zone_reclaim_stat { >> > */ >> > unsigned long recent_rotated[2]; >> > unsigned long recent_scanned[2]; >> > + >> > + /* >> > + * accumulated for batching >> > + */ >> > + unsigned long nr_saved_scan[NR_LRU_LISTS]; >> > }; >> > >> > struct zone { >> > @@ -323,7 +328,6 @@ struct zone { >> > spinlock_t lru_lock; >> > struct zone_lru { >> > struct list_head list; >> > - unsigned long nr_saved_scan; /* accumulated for batching */ >> > } lru[NR_LRU_LISTS]; >> > >> > struct zone_reclaim_stat reclaim_stat; >> > --- linux.orig/mm/vmscan.c 2009-08-20 11:48:46.000000000 +0800 >> > +++ linux/mm/vmscan.c 2009-08-20 12:00:55.000000000 +0800 >> > @@ -1521,6 +1521,7 @@ static void shrink_zone(int priority, st >> > enum lru_list l; >> > unsigned long nr_reclaimed = sc->nr_reclaimed; >> > unsigned long swap_cluster_max = sc->swap_cluster_max; >> > + struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc); >> > int noswap = 0; >> > >> > /* If we have no swap space, do not bother scanning anon pages. */ >> > @@ -1540,12 +1541,9 @@ static void shrink_zone(int priority, st >> > scan >>= priority; >> > scan = (scan * percent[file]) / 100; >> > } >> > - if (scanning_global_lru(sc)) >> > - nr[l] = nr_scan_try_batch(scan, >> > - &zone->lru[l].nr_saved_scan, >> > - swap_cluster_max); >> > - else >> > - nr[l] = scan; >> > + nr[l] = nr_scan_try_batch(scan, >> > + &reclaim_stat->nr_saved_scan[l], >> > + swap_cluster_max); >> > } >> > >> > while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] || >> > @@ -2128,6 +2126,7 @@ static void shrink_all_zones(unsigned lo >> > { >> > struct zone *zone; >> > unsigned long nr_reclaimed = 0; >> > + struct zone_reclaim_stat *reclaim_stat; >> > >> > for_each_populated_zone(zone) { >> > enum lru_list l; >> > @@ -2144,11 +2143,14 @@ static void shrink_all_zones(unsigned lo >> > l == LRU_ACTIVE_FILE)) >> > continue; >> > >> > - zone->lru[l].nr_saved_scan += (lru_pages >> prio) + 1; >> > - if (zone->lru[l].nr_saved_scan >= nr_pages || pass > 3) { >> > + reclaim_stat = get_reclaim_stat(zone, sc); >> > + reclaim_stat->nr_saved_scan[l] += >> > + (lru_pages >> prio) + 1; >> > + if (reclaim_stat->nr_saved_scan[l] >> > + >= nr_pages || pass > 3) { >> > unsigned long nr_to_scan; >> > >> > - zone->lru[l].nr_saved_scan = 0; >> > + reclaim_stat->nr_saved_scan[l] = 0; >> > nr_to_scan = min(nr_pages, lru_pages); >> > nr_reclaimed += shrink_list(l, nr_to_scan, zone, >> > sc, prio); >> > --- linux.orig/mm/page_alloc.c 2009-08-20 11:57:54.000000000 +0800 >> > +++ linux/mm/page_alloc.c 2009-08-20 11:58:39.000000000 +0800 >> > @@ -3716,7 +3716,7 @@ static void __paginginit free_area_init_ >> > zone_pcp_init(zone); >> > for_each_lru(l) { >> > INIT_LIST_HEAD(&zone->lru[l].list); >> > - zone->lru[l].nr_saved_scan = 0; >> > + zone->reclaim_stat.nr_saved_scan[l] = 0; >> > } >> > zone->reclaim_stat.recent_rotated[0] = 0; >> > zone->reclaim_stat.recent_rotated[1] = 0; >> > >> > -- >> > To unsubscribe, send a message with 'unsubscribe linux-mm' in >> > the body to majordomo@kvack.org. For more info on Linux MM, >> > see: http://www.linux-mm.org/ . >> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a> >> > >> >> >> >> -- >> Kind regards, >> Minchan Kim >
-- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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