Re: [PATCH v4] page_alloc: consider highatomic reserve in watermark fast

[Mel Gorman]

On Mon, Jun 22, 2020 at 06:40:20PM +0900, ????????? wrote:
> >But more importantly, I have hard time to follow why we need both
> >zone_watermark_fast and zone_watermark_ok now. They should be
> >essentially the same for anything but order == 0. For order 0 the
> >only difference between the two is that zone_watermark_ok checks for
> >ALLOC_HIGH resp ALLOC_HARDER, ALLOC_OOM. So what is exactly fast about
> >the former and why do we need it these days?
> > 
> 
> I think the author, Mel, may ansewr. But I think the wmark_fast may
> fast by 1) not checking more condition about wmark and 2) using inline
> rather than function. According to description on commit 48ee5f3696f6,
> it seems to bring about 4% improvement.
> 

The original intent was that watermark checks were expensive as some of the
calculations are only necessary when a zone is relatively low on memory
and the check does not always have to be 100% accurate. This is probably
still true given that __zone_watermark_ok() makes a number of calculations
depending on alloc flags even if a zone is almost completely free.

-- 
Mel Gorman
SUSE Labs