linux-kernel - Re: [PATCH] mm: use pagevec to rotate reclaimable page

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Date:	Wed, 19 Sep 2007 15:17:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: use pagevec to rotate reclaimable page

On Tue, 18 Sep 2007 19:41:14 +0900
Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp> wrote:

>  >
>  >So I do think that for safety and sanity's sake, we should be taking a ref
>  >on the pages when they are in a pagevec.  That's going to hurt your nice
>  >performance numbers :(
>  >
> 
> I did ping test again to observe performance deterioration caused by taking 
> a ref.
> 
> 	-2.6.23-rc6-with-modifiedpatch
> 	--- testmachine ping statistics ---
> 	3000 packets transmitted, 3000 received, 0% packet loss, time 53386ms
> 	rtt min/avg/max/mdev = 0.074/0.110/4.716/0.147 ms, pipe 2, ipg/ewma 
> 17.801/0.129 ms
> 
> The result for my original patch is as follows.
> 
> 	-2.6.23-rc5-with-originalpatch
> 	--- testmachine ping statistics ---
> 	3000 packets transmitted, 3000 received, 0% packet loss, time 51924ms
> 	rtt min/avg/max/mdev = 0.072/0.108/3.884/0.114 ms, pipe 2, ipg/ewma 
> 17.314/0.091 ms
> 
> 
> The influence to response was small.

well.. that's not really the test which will show up any regressions.

The extra get_page/put_page will affect things like kernel CPU utilisation
on fast writeout on a single CPU.  Say, run a huge write against a fast
storage system on a single pentium 4 CPU and see how much the system CPU
time is increased.

The kernel's internal cpu accounting probably won't be accurate enough to
get believeable numbers from a test like this - better to use the
subtractive approach: see http://www.zipworld.com.au/~akpm/linux/#zc
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