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

Andrew Morton
2007-09-19T22:18:34Z

On Tue, 18 Sep 2007 19:41:14 +0900
Hisashi Hifumi <hifumi.hisashi@oss.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