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

[Michal Hocko]

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.
-- 
Michal Hocko
SUSE Labs