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

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

On 04/03/2012 07:25 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.

Right.
As long as we consider RHEL5 users above, I believe they don't care
about cgroup 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

I made a tiny mistake.
Correct one is following ;p

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,
Satoru