On Thu 07-02-19 21:37:27, Andrew Morton wrote: > On Thu, 7 Feb 2019 11:27:50 +0100 Jan Kara <jack@xxxxxxx> wrote: > > > On Fri 01-02-19 09:19:04, Dave Chinner wrote: > > > Maybe for memcgs, but that's exactly the oppose of what we want to > > > do for global caches (e.g. filesystem metadata caches). We need to > > > make sure that a single, heavily pressured cache doesn't evict small > > > caches that lower pressure but are equally important for > > > performance. > > > > > > e.g. I've noticed recently a significant increase in RMW cycles in > > > XFS inode cache writeback during various benchmarks. It hasn't > > > affected performance because the machine has IO and CPU to burn, but > > > on slower machines and storage, it will have a major impact. > > > > Just as a data point, our performance testing infrastructure has bisected > > down to the commits discussed in this thread as the cause of about 40% > > regression in XFS file delete performance in bonnie++ benchmark. > > > > Has anyone done significant testing with Rik's maybe-fix? I will give it a spin with bonnie++ today. We'll see what comes out. Honza > > > > From: Rik van Riel <riel@xxxxxxxxxxx> > Subject: mm, slab, vmscan: accumulate gradual pressure on small slabs > > 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. > > [akpm@xxxxxxxxxxxxxxxxxxxx: whitespace fixes, per Roman] > [riel@xxxxxxxxxxx: couple of fixes] > Link: http://lkml.kernel.org/r/20190129142831.6a373403@xxxxxxxxxxxxxxxxxxxx > Link: http://lkml.kernel.org/r/20190128143535.7767c397@xxxxxxxxxxxxxxxxxxxx > Fixes: 4b85afbdacd2 ("mm: zero-seek shrinkers") > Fixes: 172b06c32b94 ("mm: slowly shrink slabs with a relatively small number of objects") > Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx> > Tested-by: Chris Mason <clm@xxxxxx> > Acked-by: Roman Gushchin <guro@xxxxxx> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Dave Chinner <dchinner@xxxxxxxxxx> > Cc: Jonathan Lemon <bsd@xxxxxx> > Cc: Jan Kara <jack@xxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> > > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > --- > > > --- a/include/linux/shrinker.h~mmslabvmscan-accumulate-gradual-pressure-on-small-slabs > +++ a/include/linux/shrinker.h > @@ -65,6 +65,7 @@ struct shrinker { > > long batch; /* reclaim batch size, 0 = default */ > int seeks; /* seeks to recreate an obj */ > + int small_scan; /* accumulate pressure on slabs with few objects */ > unsigned flags; > > /* These are for internal use */ > --- a/mm/vmscan.c~mmslabvmscan-accumulate-gradual-pressure-on-small-slabs > +++ a/mm/vmscan.c > @@ -488,18 +488,30 @@ static unsigned long do_shrink_slab(stru > * 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->seeks) { > + unsigned long nr_considered; > + > + shrinker->small_scan += freeable; > + nr_considered = shrinker->small_scan >> priority; > + > + delta = 4 * nr_considered; > + do_div(delta, shrinker->seeks); > + > + if (delta) > + shrinker->small_scan -= nr_considered << priority; > + } > > total_scan += delta; > if (total_scan < 0) { > _ > -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
