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

[Vlastimil Babka]

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
> >
> >
> > .