linux-kernel - RE: [RFC][PATCH] avoid swapping out with swappiness==0

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Message-ID: <65795E11DBF1E645A09CEC7EAEE94B9C01454D13A6@USINDEVS02.corp.hds.com>
Date:	Fri, 30 Mar 2012 18:44:27 -0400
From:	Satoru Moriya <satoru.moriya@....com>
To:	Satoru Moriya <satoru.moriya@....com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"jweiner@...hat.com" <jweiner@...hat.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"lwoodman@...hat.com" <lwoodman@...hat.com>,
	"shaohua.li@...el.com" <shaohua.li@...el.com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Seiji Aguchi <seiji.aguchi@....com>
Subject: RE: [RFC][PATCH] avoid swapping out with swappiness==0

Hello Kosaki-san,

On 03/07/2012 01:18 PM, Satoru Moriya wrote:
> On 03/07/2012 12:19 PM, KOSAKI Motohiro wrote:
>> Thank you. I brought back to memory it. Unfortunately DB folks are 
>> still mainly using RHEL5 generation distros. At that time, 
>> swapiness=0 doesn't mean disabling swap.
>>
>> They want, "don't swap as far as kernel has any file cache page". but 
>> linux don't have such feature. then they used swappiness for emulate 
>> it. So, I think this patch clearly make userland harm. Because of, we 
>> don't have an alternative way.

As I wrote in the previous mail(see below), with this patch
the kernel begins to swap out when the sum of free pages and
filebacked pages reduces less than watermark_high.

So the kernel reclaims pages like following.

nr_free + nr_filebacked >= watermark_high: reclaim only filebacked pages
nr_free + nr_filebacked <  watermark_high: reclaim only anonymous pages

Do you think this behavior satisfies DB users' requirement?


> If they expect the behavior that "don't swap as far as kernel has any 
> file cache page", this patch definitely helps them because if we set 
> swappiness==0, kernel does not swap out
> *until* nr_free + nr_filebacked < high watermark in the zone.
> It means kernel begins to swap out when nr_free + nr_filebacked 
> becomes less than high watermark.
> 
> But, yes, this patch actually changes the behavior with swappiness==0 
> and so it may make userland harm.
> 
> How about introducing new value e.g -1 to avoid swap and maintain 
> compatibility?

Regards,
Satoru
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