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Message-ID: <5791E5B1.8060503@huawei.com>
Date: Fri, 22 Jul 2016 17:21:53 +0800
From: Xishi Qiu <qiuxishi@huawei.com>
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To: Vlastimil Babka <vbabka@suse.cz>
CC: "mel@csn.ul.ie" <mel@csn.ul.ie>, David Rientjes <rientjes@google.com>,
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        Mel Gorman <mgorman@techsingularity.net>,
        Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: mm/compact: why use low watermark to determine whether compact
 is finished instead of use high watermark?
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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
>>
> 
> 
> .
>