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

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Date:   Mon, 22 Jun 2020 11:04:39 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     ????????? <jaewon31.kim@...sung.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "bhe@...hat.com" <bhe@...hat.com>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jaewon31.kim@...il.com" <jaewon31.kim@...il.com>,
        ????????? <ytk.lee@...sung.com>,
        ????????? <cmlaika.kim@...sung.com>
Subject: Re: [PATCH v4] page_alloc: consider highatomic reserve in watermark
 fast

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