linux-kernel - Re: mm/compact: why use low watermark to determine whether compact is finished instead of use high watermark?

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Message-ID: <0b580155-d99a-f4a4-ef76-6166b41180aa@suse.cz>
Date:	Fri, 22 Jul 2016 11:04:20 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Xishi Qiu <qiuxishi@...wei.com>, "mel@....ul.ie" <mel@....ul.ie>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <js1304@...il.com>,
	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: mm/compact: why use low watermark to determine whether compact is
 finished instead of use high watermark?

On 07/22/2016 10:56 AM, Xishi Qiu wrote:
> Hi,
>
> I find all the watermarks in mm/compaction.c are low_wmark_pages(),
> so why not use high watermark to determine whether compact is finished?

Why would you use high watermark? Quite the opposite, I want to move 
towards min watermark (precisely, the one in alloc_flags which is 
usually min) in this series:

https://lkml.org/lkml/2016/6/24/222

especially:

https://lkml.org/lkml/2016/6/24/214

> e.g.
> __alloc_pages_nodemask()
> 	get_page_from_freelist()
> 	this is fast path, use use low_wmark_pages() in __zone_watermark_ok()
>
> 	__alloc_pages_slowpath()
> 	this is slow path, usually use min_wmark_pages()

Yes, and compaction should be finished when allocation can succeed, so 
match __alloc_pages_slowpath().

>
> kswapd
> 	balance_pgdat()
> 	use high_wmark_pages() to determine whether zone is balanced
>
> Thanks,
> Xishi Qiu
>