linux-kernel - Re: [RFC PATCH 4/4] mm: Add PG

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Date:   Wed, 22 Apr 2020 16:09:00 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [RFC PATCH 4/4] mm: Add PG_zero support

On 4/13/20 11:05 PM, Andrew Morton wrote:
> On Mon, 13 Apr 2020 08:11:59 -0700 Alexander Duyck <alexander.h.duyck@...ux.intel.com> wrote:
> 
>> In addition, unlike madvising the page away there is a pretty 
>> significant performance penalty for having to clear the page a second 
>> time when the page is split or merged.
> 
> I wonder if there might be an issue with increased memory traffic (and
> increased energy consumption, etc).  If a page is zeroed immediately
> before getting data written into it (eg, plain old file write(),
> anonymous pagefault) then we can expect that those 4096 zeroes will be
> in CPU cache and mostly not written back.  But if that page was zeroed
> a "long" time ago, the caches will probably have been written back. 
> Net result: we go from 4k of memory traffic for a 4k page up to 8k of
> memory traffic?

Heh, I was quite sure that this is not the first time background zeroing is
proposed, so I went to google for it... and found that one BSD kernel actually
removed this functionality in 2016 [1] and this was one of the reasons.

[1]
https://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/afd2da4dc9056ea79cdf15e8a9386a3d3998f33e

> Also, the name CONFIG_ZERO_PAGE sounds like it has something to do with
> the long established "zero page".  Confusing.  CONFIG_PREZERO_PAGE,
> maybe?
>