linux-kernel - Re: [PATCH 0/3] OOM detection rework v4

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Date:	Thu, 3 Mar 2016 16:50:16 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <js1304@...il.com>, Michal Hocko <mhocko@...nel.org>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Hillf Danton <hillf.zj@...baba-inc.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 0/3] OOM detection rework v4

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

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