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

From	Xishi Qiu <qiuxishi@huawei.com>
Newsgroups	linux.kernel
Subject	mm/compact: why use low watermark to determine whether compact is finished instead of use high watermark?
Date	2016-07-22 11:10 +0200
Message-ID	<rXED0-3Mr-19@gated-at.bofh.it> (permalink)
Organization	linux.* mail to news gateway

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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?

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()

kswapd
	balance_pgdat()
	use high_wmark_pages() to determine whether zone is balanced

Thanks,
Xishi Qiu

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mm/compact: why use low watermark to determine whether compact is  finished instead of use high watermark? Xishi Qiu <qiuxishi@huawei.com> - 2016-07-22 11:10 +0200
  Re: mm/compact: why use low watermark to determine whether compact is  finished instead of use high watermark? Vlastimil Babka <vbabka@suse.cz> - 2016-07-22 11:10 +0200
    Re: mm/compact: why use low watermark to determine whether compact  is finished instead of use high watermark? Xishi Qiu <qiuxishi@huawei.com> - 2016-07-22 11:40 +0200
      Re: mm/compact: why use low watermark to determine whether compact is  finished instead of use high watermark? Vlastimil Babka <vbabka@suse.cz> - 2016-07-22 11:40 +0200

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