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

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Date:	Mon, 01 Sep 2008 14:47:26 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Pavel Emelyanov <xemul@...nvz.org>
CC:	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

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).

-- 
	Balbir
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