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

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Date:	Fri, 22 Jul 2016 17:21:53 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Vlastimil Babka <vbabka@...e.cz>
CC:	"mel@....ul.ie" <mel@....ul.ie>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <js1304@...il.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	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 2016/7/22 17:04, Vlastimil Babka wrote:

> 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().
> 

Sounds reasonable, but now we have kcompactd which called from kswapd,
so still use low wmark?

Thanks,
Xishi Qiu

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