Date: Thu, 3 Mar 2016 17:26:28 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Joonsoo Kim <js1304@gmail.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>, Mel Gorman <mgorman@suse.de>,
        David Rientjes <rientjes@google.com>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Hillf Danton <hillf.zj@alibaba-inc.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [PATCH 0/3] OOM detection rework v4
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On Thu 03-03-16 16:50:16, Vlastimil Babka wrote:
> On 03/03/2016 03:10 PM, Joonsoo Kim wrote:
> > 
> >> [...]
> >>>>> At least, reset no_progress_loops when did_some_progress. High
> >>>>> order allocation up to PAGE_ALLOC_COSTLY_ORDER is as important
> >>>>> as order 0. And, reclaim something would increase probability of
> >>>>> compaction success.
> >>>>
> >>>> This is something I still do not understand. Why would reclaiming
> >>>> random order-0 pages help compaction? Could you clarify this please?
> >>>
> >>> I just can tell simple version. Please check the link from me on another reply.
> >>> Compaction could scan more range of memory if we have more freepage.
> >>> This is due to algorithm limitation. Anyway, so, reclaiming random
> >>> order-0 pages helps compaction.
> >>
> >> I will have a look at that code but this just doesn't make any sense.
> >> The compaction should be reshuffling pages, this shouldn't be a function
> >> of free memory.
> > 
> > Please refer the link I mentioned before. There is a reason why more free
> > memory would help compaction success. Compaction doesn't work
> > like as random reshuffling. It has an algorithm to reduce system overall
> > fragmentation so there is limitation.
> 
> I proposed another way to get better results from direct compaction -
> don't scan for free pages but get them directly from freelists:
> 
> https://lkml.org/lkml/2015/12/3/60

Yes this makes perfect sense to me (with my limited experience in
this area so I might be missing some obvious problems this would
introduce). The direct compaction for !costly orders is something
we should better satisfy immediately. I would just object that this
shouldn't be reduced to ASYNC compaction requests only. SYNC* modes are
even a more desperate call (at least that is my understanding) for the
page and we should treat them the appropriately.

> But your redesign would be useful too for kcompactd/khugepaged keeping
> overall fragmentation low.

kcompactd can handle and should focus on the long term goals.

-- 
Michal Hocko
SUSE Labs