Re: commit 0bf1457f0cfca7b " mm: vmscan: do not swap anon pages just because free+file is low" causes heavy performance regression on paging

Johannes Weiner
2014-04-24T12:24:18Z

Hi Rik,

On Tue, Apr 22, 2014 at 10:40:17AM -0400, Rik van Riel wrote:
> On 04/22/2014 07:57 AM, Christian Borntraeger wrote:
> > On 22/04/14 12:55, Christian Borntraeger wrote:
> >> While preparing/testing some KVM on s390 patches for the next merge window (target is kvm/next which is based on 3.15-rc1) I faced a very severe performance hickup on guest paging (all anonymous memory).
> >>
> >> All memory bound guests are in "D" state now and the system is barely unusable.
> >>
> >> Reverting commit 0bf1457f0cfca7bc026a82323ad34bcf58ad035d
> >> "mm: vmscan: do not swap anon pages just because free+file is low" makes the problem go away.
> >>
> >> According to /proc/vmstat the system is now in direct reclaim almost all the time for every page fault (more than 10x more direct reclaims than kswap reclaims)
> >> With the patch being reverted everything is fine again.
> >>
> >> Any ideas?
> >
> > Here is an idea to tackle my problem and the original problem:
> >
> > reverting 0bf1457f0cfca7bc026a82323ad34bcf58ad035d + checking against low, also seems to make my system usable.
> >
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1923,7 +1923,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
> > */
> > if (global_reclaim(sc)) {
> > free = zone_page_state(zone, NR_FREE_PAGES);
> > - if (unlikely(file + free <= high_wmark_pages(zone))) {
> > + if (unlikely(file + free <= low_wmark_pages(zone))) {
> > scan_balance = SCAN_ANON;
> > goto out;
> > }
> >
>
> Looks reasonable to me. Johannes?

I went with a full revert to be on the safe side. Since kswapd's goal
is the high watermark, I kind of liked the idea that we start swapping
once the file pages alone are not enough anymore to restore the wmark.