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

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Date:	Thu, 18 Sep 2008 13:26:13 +0900 (JST)
From:	Hirokazu Takahashi <taka@...inux.co.jp>
To:	akpm@...ux-foundation.org
Cc:	balbir@...ux.vnet.ibm.com, kamezawa.hiroyu@...fujitsu.com,
	dave@...ux.vnet.ibm.com, 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)

Hi,

> > 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.

I think this design is just temporary and the goal is to pre-allocate
all page_cgroups at boot time if it isn't disabled.

But I think each memory model type should have its own way of managing
its page_cgroup arrays as doing for its struct page arrays.
It would be better rather than the sparsemem approach he said.

> 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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