Re: [RFC PATCH 4/4] mm: Add PG_zero support

[Vlastimil Babka]

On 4/13/20 11:05 PM, Andrew Morton wrote:
> On Mon, 13 Apr 2020 08:11:59 -0700 Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> 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?
>