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

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Message-Id: <20200413140537.eb674579cf8c71b4e20581ab@linux-foundation.org>
Date:   Mon, 13 Apr 2020 14:05:37 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     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 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?

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