On Mon, 28 Jan 2019 14:35:35 -0500 Rik van Riel <riel@xxxxxxxxxxx> wrote:
> There are a few issues with the way the number of slab objects to
> scan is calculated in do_shrink_slab. First, for zero-seek slabs,
> we could leave the last object around forever. That could result
> in pinning a dying cgroup into memory, instead of reclaiming it.
> The fix for that is trivial.
>
> Secondly, small slabs receive much more pressure, relative to their
> size, than larger slabs, due to "rounding up" the minimum number of
> scanned objects to batch_size.
>
> We can keep the pressure on all slabs equal relative to their size
> by accumulating the scan pressure on small slabs over time, resulting
> in sometimes scanning an object, instead of always scanning several.
>
> This results in lower system CPU use, and a lower major fault rate,
> as actively used entries from smaller caches get reclaimed less
> aggressively, and need to be reloaded/recreated less often.
>
> Fixes: 4b85afbdacd2 ("mm: zero-seek shrinkers")
> Fixes: 172b06c32b94 ("mm: slowly shrink slabs with a relatively small number of objects")
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Chris Mason <clm@xxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>
> Cc: kernel-team@xxxxxx
> Tested-by: Chris Mason <clm@xxxxxx>
I added your Signed-off-by:
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -488,18 +488,28 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> * them aggressively under memory pressure to keep
> * them from causing refetches in the IO caches.
> */
> - delta = freeable / 2;
> + delta = (freeable + 1)/ 2;
> }
>
> /*
> * Make sure we apply some minimal pressure on default priority
> - * even on small cgroups. Stale objects are not only consuming memory
> + * even on small cgroups, by accumulating pressure across multiple
> + * slab shrinker runs. Stale objects are not only consuming memory
> * by themselves, but can also hold a reference to a dying cgroup,
> * preventing it from being reclaimed. A dying cgroup with all
> * corresponding structures like per-cpu stats and kmem caches
> * can be really big, so it may lead to a significant waste of memory.
> */
> - delta = max_t(unsigned long long, delta, min(freeable, batch_size));
> + if (!delta) {
> + shrinker->small_scan += freeable;
> +
> + delta = shrinker->small_scan >> priority;
> + shrinker->small_scan -= delta << priority;
> +
> + delta *= 4;
> + do_div(delta, shrinker->seeks);
What prevents shrinker->small_scan from over- or underflowing over time?
> + }
>
> total_scan += delta;
> if (total_scan < 0) {
I'll add this:
whitespace fixes, per Roman
--- a/mm/vmscan.c~mmslabvmscan-accumulate-gradual-pressure-on-small-slabs-fix
+++ a/mm/vmscan.c
@@ -488,7 +488,7 @@ static unsigned long do_shrink_slab(stru
* them aggressively under memory pressure to keep
* them from causing refetches in the IO caches.
*/
- delta = (freeable + 1)/ 2;
+ delta = (freeable + 1) / 2;
}
/*
@@ -508,7 +508,6 @@ static unsigned long do_shrink_slab(stru
delta *= 4;
do_div(delta, shrinker->seeks);
-
}
total_scan += delta;
_
