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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, Jul 2, 2018 at 3:34 PM James Bottomley
<James.Bottomley@hansenpartnership.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