linux-kernel - Re: [RFC][PATCH 0/2] Tunable watermark

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Date:	Thu, 20 Jan 2011 19:16:48 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Satoru Moriya <satoru.moriya@....com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mel@....ul.ie" <mel@....ul.ie>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"rdunlap@...otime.net" <rdunlap@...otime.net>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Seiji Aguchi <seiji.aguchi@....com>
Subject: Re: [RFC][PATCH 0/2] Tunable watermark

On 01/07/2011 05:03 PM, Satoru Moriya wrote:

> The result is following.
>
>                    | default |  case 1   |  case 2 |
> ----------------------------------------------------------
> wmark_min_kbytes  |  5752   |    5752   |   5752  |
> wmark_low_kbytes  |  7190   |   16384   |  32768  | (KB)
> wmark_high_kbytes |  8628   |   20480   |  40960  |
> ----------------------------------------------------------
> real              |   503   |    364    |    337  |
> user              |     3   |      5    |      4  | (msec)
> sys               |   153   |    149    |    146  |
> ----------------------------------------------------------
> page fault        |  32768  |  32768    |  32768  |
> kswapd_wakeup     |   1809  |    335    |    228  | (times)
> direct reclaim    |      5  |      0    |      0  |
>
> As you can see, direct reclaim was performed 5 times and
> its exec time was 503 msec in the default case. On the other
> hand, in case 1 (large delta case ) no direct reclaim was
> performed and its exec time was 364 msec.

Saving 1.5 seconds on a one-off workload is probably not
worth the complexity of giving a system administrator
yet another set of tunables to mess with.

However, I suspect it may be a good idea if the kernel
could adjust these watermarks automatically, since direct
reclaim could lead to quite a big performance penalty.

I do not know which events should be used to increase and
decrease the watermarks, but I have some ideas:
- direct reclaim (increase)
- kswapd has trouble freeing pages (increase)
- kswapd frees enough memory at DEF_PRIORITY (decrease)
- next to no direct reclaim events in the last N (1000?)
   reclaim events (decrease)

I guess we will also need to be sure that the watermarks
are never raised above some sane upper threshold.  Maybe
4x or 5x the default?


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