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

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Message-ID: <20180716090901.GG17280@dhcp22.suse.cz>
Date:   Mon, 16 Jul 2018 11:09:01 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     James Bottomley <James.Bottomley@...senPartnership.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Waiman Long <longman@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        "Wangkai (Kevin,C)" <wangkai86@...wei.com>
Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries

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