Re: [patch 2/8] mm: memcg-aware global reclaim

Minchan Kim
2011-06-10T00:48:40Z

On Fri, Jun 10, 2011 at 9:34 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Fri, Jun 10, 2011 at 08:47:55AM +0900, Minchan Kim wrote:
>> On Fri, Jun 10, 2011 at 8:41 AM, Minchan Kim <minchan.kim@gmail.com> wrote:
>> > On Fri, Jun 10, 2011 at 2:23 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
>> >> On Fri, Jun 10, 2011 at 12:48:39AM +0900, Minchan Kim wrote:
>> >>> On Wed, Jun 01, 2011 at 08:25:13AM +0200, Johannes Weiner wrote:
>> >>> > When a memcg hits its hard limit, hierarchical target reclaim is
>> >>> > invoked, which goes through all contributing memcgs in the hierarchy
>> >>> > below the offending memcg and reclaims from the respective per-memcg
>> >>> > lru lists.  This distributes pressure fairly among all involved
>> >>> > memcgs, and pages are aged with respect to their list buddies.
>> >>> >
>> >>> > When global memory pressure arises, however, all this is dropped
>> >>> > overboard.  Pages are reclaimed based on global lru lists that have
>> >>> > nothing to do with container-internal age, and some memcgs may be
>> >>> > reclaimed from much more than others.
>> >>> >
>> >>> > This patch makes traditional global reclaim consider container
>> >>> > boundaries and no longer scan the global lru lists.  For each zone
>> >>> > scanned, the memcg hierarchy is walked and pages are reclaimed from
>> >>> > the per-memcg lru lists of the respective zone.  For now, the
>> >>> > hierarchy walk is bounded to one full round-trip through the
>> >>> > hierarchy, or if the number of reclaimed pages reach the overall
>> >>> > reclaim target, whichever comes first.
>> >>> >
>> >>> > Conceptually, global memory pressure is then treated as if the root
>> >>> > memcg had hit its limit.  Since all existing memcgs contribute to the
>> >>> > usage of the root memcg, global reclaim is nothing more than target
>> >>> > reclaim starting from the root memcg.  The code is mostly the same for
>> >>> > both cases, except for a few heuristics and statistics that do not
>> >>> > always apply.  They are distinguished by a newly introduced
>> >>> > global_reclaim() primitive.
>> >>> >
>> >>> > One implication of this change is that pages have to be linked to the
>> >>> > lru lists of the root memcg again, which could be optimized away with
>> >>> > the old scheme.  The costs are not measurable, though, even with
>> >>> > worst-case microbenchmarks.
>> >>> >
>> >>> > As global reclaim no longer relies on global lru lists, this change is
>> >>> > also in preparation to remove those completely.
>> >>
>> >> [cut diff]
>> >>
>> >>> I didn't look at all, still. You might change the logic later patches.
>> >>> If I understand this patch right, it does round-robin reclaim in all memcgs
>> >>> when global memory pressure happens.
>> >>>
>> >>> Let's consider this memcg size unbalance case.
>> >>>
>> >>> If A-memcg has lots of LRU pages, scanning count for reclaim would be bigger
>> >>> so the chance to reclaim the pages would be higher.
>> >>> If we reclaim A-memcg, we can reclaim the number of pages we want easily and break.
>> >>> Next reclaim will happen at some time and reclaim will start the B-memcg of A-memcg
>> >>> we reclaimed successfully before. But unfortunately B-memcg has small lru so
>> >>> scanning count would be small and small memcg's LRU aging is higher than bigger memcg.
>> >>> It means small memcg's working set can be evicted easily than big memcg.
>> >>> my point is that we should not set next memcg easily.
>> >>> We have to consider memcg LRU size.
>> >>
>> >> I may be missing something, but you said yourself that B had a smaller
>> >> scan count compared to A, so the aging speed should be proportional to
>> >> respective size.
>> >>
>> >> The number of pages scanned per iteration is essentially
>> >>
>> >>        number of lru pages in memcg-zone >> priority
>> >>
>> >> so we scan relatively more pages from B than from A each round.
>> >>
>> >> It's the exact same logic we have been applying traditionally to
>> >> distribute pressure fairly among zones to equalize their aging speed.
>> >>
>> >> Is that what you meant or are we talking past each other?
>> >
>> > True if we can reclaim pages easily(ie, default priority) in all memcgs.
>> > But let's think about it.
>> > Normally direct reclaim path reclaims only SWAP_CLUSTER_MAX size.
>> > If we have small memcg, scan window size would be smaller and it is
>> > likely to be hard reclaim in the priority compared to bigger memcg. It
>> > means it can raise priority easily in small memcg and even it might
>> > call lumpy or compaction in case of global memory pressure. It can
>> > churn all LRU order. :(
>> > Of course, we have bailout routine so we might make such unfair aging
>> > effect small but it's not same with old behavior(ie, single LRU list,
>> > fair aging POV global according to priority raise)
>>
>> To make fair, how about considering turn over different memcg before
>> raise up priority?
>> It can make aging speed fairly while it can make high contention of
>> lru_lock. :(
>
> Actually, the way you describe it is how it used to work for limit
> reclaim before my patches.  It would select one memcg, then reclaim
> with increasing priority until SWAP_CLUSTER_MAX were reclaimed.
>
>        memcg = select_victim()
>        for each prio:
>          for each zone:
>            shrink_zone(prio, zone, sc = { .mem_cgroup = memcg })
>
> What it's supposed to do with my patches is scan all memcgs in the
> hierarchy at the same priority.  If it hasn't made progress, it will
> increase the priority and iterate again over the hierarchy.
>
>        for each prio:
>          for each zone:
>            for each memcg:
>              do_shrink_zone(prio, zone, sc = { .mem_cgroup = memcg })
>
>

Right you are. I got confused with old behavior which wasn't good.
Your way is very desirable to me and my concern disappear.
Thanks, Hannes.

--
Kind regards,
Minchan Kim