Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries

[Michal Hocko]

On Wed 18-07-18 12:17:24, Waiman Long wrote:
> On 07/16/2018 05:09 AM, Michal Hocko wrote:
> > On Fri 13-07-18 10:36:14, Dave Chinner wrote:
> > [...]
> >> By limiting the number of negative dentries in this case, internal
> >> slab fragmentation is reduced such that reclaim cost never gets out
> >> of control. While it appears to "fix" the symptoms, it doesn't
> >> address the underlying problem. It is a partial solution at best but
> >> at worst it's another opaque knob that nobody knows how or when to
> >> tune.
> > Would it help to put all the negative dentries into its own slab cache?
> >
> >> Very few microbenchmarks expose this internal slab fragmentation
> >> problem because they either don't run long enough, don't create
> >> memory pressure, or don't have access patterns that mix long and
> >> short term slab objects together in a way that causes slab
> >> fragmentation. Run some cold cache directory traversals (git
> >> status?) at the same time you are creating negative dentries so you
> >> create pinned partial pages in the slab cache and see how the
> >> behaviour changes....
> > Agreed! Slab fragmentation is a real problem we are seeing for quite
> > some time. We should try to address it rather than paper over it with
> > weird knobs.
> 
> I am aware that you don't like the limit knob that control how many
> negative dentries are allowed as a percentage of total system memory. I
> got comments in the past about doing some kind of auto-tuning. How about
> consolidating the 2 knobs that I currently have in the patchset into a
> single one with 3 possible values, like:
> 
> 0 - no limiting
> 1 - set soft limit to "a constant + 4 x max # of positive dentries" and
> warn if exceeded
> 2 - same limit but kill excess negative dentries after use.
> 
> Does that kind of knob make more sense to you?

Not really. See the pagecache limit story in http://lkml.kernel.org/r/20180719084538.GP7193@xxxxxxxxxxxxxx
I might be overly sensitive but I got burnt a lot in the past. We should
strive to make the reclaim seamless without asking admins to do our job.
-- 
Michal Hocko
SUSE Labs