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

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

(4/3/12 4:25 AM), Jerome Marchand wrote:
> On 04/02/2012 07:10 PM, KOSAKI Motohiro wrote:
>> 2012/3/30 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.
>
> Actually, this is true only for global reclaims. Reclaims in cgroup can fail
> in this case.
>
>>>
>>> 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
>>
>> How?
>
> get_scan_count() checks that case explicitly:
>
> 	if (global_reclaim(sc)) {
> 		free  = zone_page_state(mz->zone, NR_FREE_PAGES);
> 		/* If we have very few page cache pages,
> 		   force-scan anon pages. */
> 		if (unlikely(file + free<= high_wmark_pages(mz->zone))) {
> 			fraction[0] = 1;
> 			fraction[1] = 0;
> 			denominator = 1;
> 			goto out;
> 		}
> 	}

Eek. This is silly. Nowaday many people enabled THP and it increase zone watermark.
so, high watermask is not good threshold anymore.





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