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

[Christian Borntraeger <borntraeger@de.ibm.com>]

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;
                }