linux-kernel - Re: [RFC][PATCH] Remove cgroup member from struct page

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Message-Id: <1220275162.8426.61.camel@twins>
Date:	Mon, 01 Sep 2008 15:19:22 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	balbir@...ux.vnet.ibm.com
Cc:	Pavel Emelyanov <xemul@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>, hugh@...itas.com,
	kamezawa.hiroyu@...fujitsu.com, menage@...gle.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH] Remove cgroup member from struct page

On Mon, 2008-09-01 at 14:47 +0530, Balbir Singh wrote:
> Pavel Emelyanov wrote:
> > Balbir Singh wrote:
> >> This is a rewrite of a patch I had written long back to remove struct page
> >> (I shared the patches with Kamezawa, but never posted them anywhere else).
> >> I spent the weekend, cleaning them up for 2.6.27-rc5-mmotm (29 Aug 2008).
> >>
> >> I've tested the patches on an x86_64 box, I've run a simple test running
> >> under the memory control group and the same test running concurrently under
> >> two different groups (and creating pressure within their groups). I've also
> >> compiled the patch with CGROUP_MEM_RES_CTLR turned off.
> >>
> >> Advantages of the patch
> >>
> >> 1. It removes the extra pointer in struct page
> >>
> >> Disadvantages
> >>
> >> 1. It adds an additional lock structure to struct page_cgroup
> >> 2. Radix tree lookup is not an O(1) operation, once the page is known
> >>    getting to the page_cgroup (pc) is a little more expensive now.
> > 
> > And besides, we also have a global lock, that protects even lookup
> > from this structure. Won't this affect us too much on bug-smp nodes?
> 
> Sorry, not sure I understand. The lookup is done under RCU. Updates are done
> using the global lock. It should not be hard to make the radix tree per node
> later (as an iterative refinement).

Or you could have a look at the concurrent radix tree, esp for dense
trees it can save a lot on lock bouncing.

Latest code available at:

http://programming.kicks-ass.net/kernel-patches/concurrent-pagecache/27-rc3/

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