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Date: Mon, 16 Jul 2018 11:09:01 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries
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To: Dave Chinner <david@fromorbit.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>, Linus Torvalds <torvalds@linux-foundation.org>, Matthew Wilcox <willy@infradead.org>, Waiman Long <longman@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>, Jonathan Corbet <corbet@lwn.net>, "Luis R. Rodriguez" <mcgrof@kernel.org>, Kees Cook <keescook@chromium.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>, Jan Kara <jack@suse.cz>, Paul McKenney <paulmck@linux.vnet.ibm.com>, Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@kernel.org>, Miklos Szeredi <mszeredi@redhat.com>, Larry Woodman <lwoodman@redhat.com>, "Wangkai (Kevin,C)" <wangkai86@huawei.com>

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