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

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Date:	Mon, 3 Nov 2014 23:52:06 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Johannes Weiner <hannes@...xchg.org>
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 04:36:28PM -0500, Johannes Weiner wrote:
> 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).

I read the description. I want to know if runtime win (any benchmark data?)
from moving mem_cgroup back to the struct page is measurable.

If the win is not significant, I would prefer to not occupy the padding:
I'm sure we will be able to find a better use for the space in struct page
in the future.

-- 
 Kirill A. Shutemov
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