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

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Message-Id: <20080918134304.93985542.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 18 Sep 2008 13:43:04 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	balbir@...ux.vnet.ibm.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 18:40:08 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 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? ;)
> 

Balbir, don't you like pre-allocate-page-cgroup-at-boot at all ?
I don't like radix-tree for objects which can spread to very vast/sparse area ;)

BTW, I already have lazy-lru-by-pagevec protocol on my patch(hash version) and
seems to work well. I'm now testing it and will post today if I'm enough lucky.

Thanks,
-Kame

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