linux-kernel - Re: [patch 1/3] mm: embed the memcg pointer directly into struct page

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Message-ID: <20141103213628.GA11428@phnom.home.cmpxchg.org>
Date:	Mon, 3 Nov 2014 16:36:28 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Tejun Heo <tj@...nel.org>, David Miller <davem@...emloft.net>,
	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/3] mm: embed the memcg pointer directly into struct page

On Mon, Nov 03, 2014 at 11:06:07PM +0200, Kirill A. Shutemov wrote:
> On Sat, Nov 01, 2014 at 11:15:54PM -0400, Johannes Weiner wrote:
> > Memory cgroups used to have 5 per-page pointers.  To allow users to
> > disable that amount of overhead during runtime, those pointers were
> > allocated in a separate array, with a translation layer between them
> > and struct page.
> > 
> > There is now only one page pointer remaining: the memcg pointer, that
> > indicates which cgroup the page is associated with when charged.  The
> > complexity of runtime allocation and the runtime translation overhead
> > is no longer justified to save that *potential* 0.19% of memory.
> 
> How much do you win by the change?

Heh, that would have followed right after where you cut the quote:
with CONFIG_SLUB, that pointer actually sits in already existing
struct page padding, which means that I'm saving one pointer per page
(8 bytes per 4096 byte page, 0.19% of memory), plus the pointer and
padding in each memory section.  I also save the (minor) translation
overhead going from page to page_cgroup and the maintenance burden
that stems from having these auxiliary arrays (see deleted code).
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