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 11:30:12 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Xishi Qiu <qiuxishi@...wei.com>
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 07/22/2016 11:21 AM, Xishi Qiu wrote:
> 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?

kswapd uses high watermark (order-0) to make a buffer of free pages for 
both direct allocators and kcompactd. kcompactd will use min watermark 
for now. There used to be difference between min/low/high watermarks 
when checking high-orders, but now watermark only affects the check if 
there's enough total base pages, and high-order check then succeeds if 
there's at least one high(er)-order page.

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