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

[Jerome Marchand <jmarchan@redhat.com>]

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;
		}
	}

Regards,
Jerome

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