linux-kernel - Re: [RFC][PATCH] Remove cgroup member from struct page (v3)

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Date:	Wed, 17 Sep 2008 18:40:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	balbir@...ux.vnet.ibm.com
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, hugh@...itas.com,
	menage@...gle.com, xemul@...nvz.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [RFC][PATCH] Remove cgroup member from struct page (v3)

On Wed, 17 Sep 2008 16:28:26 -0700 Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> 
> Before trying the sparsemem approach, I tried a radix tree per node,
> per zone and I seemed to actually get some performance
> improvement.(1.5% (noise maybe))
> 
> But please do see and review (tested on my x86_64 box with unixbench
> and some other simple tests)
> 
> v4..v3
> 1. Use a radix tree per node, per zone
> 
> v3...v2
> 1. Convert flags to unsigned long
> 2. Move page_cgroup->lock to a bit spin lock in flags
> 
> v2...v1
> 
> 1. Fix a small bug, don't call radix_tree_preload_end(), if preload fails
> 
> 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).
> 
> Advantages of the patch
> 
> 1. It removes the extra pointer in struct page
> 
> Disadvantages
> 
> 1. 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.

Why are we doing this?  I can guess, but I'd rather not have to.

a) It's slower.

b) It uses even more memory worst-case.

c) It uses less memory best-case.

someone somewhere decided that (Aa + Bb) / Cc < 1.0.  What are the values
of A, B and C and where did they come from? ;)

(IOW, your changelog is in the category "sucky", along with 90% of the others)
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