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

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Message-ID: <48D1DFE0.5010208@linux.vnet.ibm.com>
Date:	Wed, 17 Sep 2008 21:58:08 -0700
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	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)

KAMEZAWA Hiroyuki wrote:
> 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 ;)
> 

I tried one version, but before trying a pre-allocation version, I wanted to
spread out the radix-tree and try and the results seemed quite impressive. We
can still do pre-allocation, but it gets more complicated as we start supporting
all memory models. I do have a design on paper, but it is much more complex than
this.

> 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.

cool! Please do post what numbers you see as well. I would appreciate if you can
try this version and see what sort of performance issues you see.

-- 
	Balbir
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