[Devel] [PATCH RHEL7 COMMIT] vmscan: fix slab vs lru balance

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-229.7.2.vz7.9.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rebase
------>
commit a9c05ec9835f2b4fda163cbf676f89b1c72fe28e
Author: Vladimir Davydov <vdavydov at virtuozzo.com>
Date:   Wed Dec 2 19:09:47 2015 +0400

    vmscan: fix slab vs lru balance
    
    The comment to shrink_slab states that the portion of kmem objects
    scanned by it equals the portion of lru pages scanned by shrink_zone
    over shrinker->seeks.
    
    shrinker->seeks is supposed to be equal to the number of disk seeks
    required to recreated an object. It is usually set to DEFAULT_SEEKS (2),
    which is quite logical, because most kmem objects (e.g. dentry or inode)
    require random IO to reread (seek to read and seek back).
    
    That said, one would expect that dcache is scanned two times less
    intensively than page cache, which sounds sane as dentries are generally
    more costly to recreate.
    
    However, the formula for distributing memory pressure between slab and
    lru actually looks as follows (see do_shrink_slab):
    
                                  lru_scanned
    objs_to_scan = objs_total * --------------- * 4 / shrinker->seeks
                                lru_reclaimable
    
    That is dcache, as well as most of other slab caches, is scanned two
    times more aggressively than page cache.
    
    Fix this by dropping '4' from the equation above.
    
    Signed-off-by: Vladimir Davydov <vdavydov at virtuozzo.com>
    Reviewed-by: Kirill Tkhai <ktkhai at odin.com>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9b1c73b..eadc08b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -262,7 +262,7 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 	nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
 
 	total_scan = nr;
-	delta = (4 * nr_scanned) / shrinker->seeks;
+	delta = nr_scanned / shrinker->seeks;
 	delta *= max_pass;
 	do_div(delta, nr_eligible + 1);
 	total_scan += delta;