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

[Vlastimil Babka]

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