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

[Satoru Moriya <satoru.moriya@hds.com>]

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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