linux-kernel - Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries

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Date:   Mon, 2 Jul 2018 15:54:04 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     James Bottomley <James.Bottomley@...senpartnership.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Waiman Long <longman@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Larry Woodman <lwoodman@...hat.com>,
        "Wangkai (Kevin,C)" <wangkai86@...wei.com>,
        linux-mm <linux-mm@...ck.org>, Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries

On Mon, Jul 2, 2018 at 3:34 PM James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
>
> There are still a lot of applications that keep looking up non-existent
> files, so I think it's still beneficial to keep them.  Apparently
> apache still looks for a .htaccess file in every directory it
> traverses, for instance.

.. or git looking for ".gitignore" files in every directory, or any
number of similar things.

Lookie here, for example:

  [torvalds@i7 linux]$ strace -e trace=%file -c git status
  On branch master
  Your branch is up to date with 'origin/master'.

  nothing to commit, working tree clean
  % time     seconds  usecs/call     calls    errors syscall
  ------ ----------- ----------- --------- --------- ----------------
   73.23    0.009066           2      4056         6 open
   23.33    0.002888           2      1294      1189 openat
    1.60    0.000198          13        15         8 access
    0.80    0.000099           2        36        31 lstat
    0.53    0.000066           1        40         6 stat
    0.27    0.000033           8         4           getcwd
    0.11    0.000014          14         1           execve
    0.11    0.000014          14         1           chdir
    0.02    0.000003           3         1         1 readlink
    0.00    0.000000           0         1           unlink
  ------ ----------- ----------- --------- --------- ----------------
  100.00    0.012381                  5449      1241 total

so almost a quarter (1241 of 5449) of the file accesses resulted in
errors (and I think they are all ENOENT).

Negative lookups are *not* some unusual thing.

                   Linus