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Subject: Re: [PATCH 00/11] fs/dcache: Limit # of negative dentries
From:   Ian Kent <raven@themaw.net>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andreas Dilger <adilger@dilger.ca>,
        Waiman Long <longman@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        Luis Chamberlain <mcgrof@kernel.org>,
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        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Eric Biggers <ebiggers@google.com>,
        Dave Chinner <david@fromorbit.com>,
        Eric Sandeen <sandeen@redhat.com>
Date:   Fri, 28 Feb 2020 12:36:09 +0800
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On Fri, 2020-02-28 at 12:16 +0800, Ian Kent wrote:
> On Thu, 2020-02-27 at 19:34 -0800, Matthew Wilcox wrote:
> > On Thu, Feb 27, 2020 at 05:55:43PM +0800, Ian Kent wrote:
> > > Not all file systems even produce negative hashed dentries.
> > > 
> > > The most beneficial use of them is to improve performance of
> > > rapid
> > > fire lookups for non-existent names. Longer lived negative hashed
> > > dentries don't give much benefit at all unless they suddenly have
> > > lots of hits and that would cost a single allocation on the first
> > > lookup if the dentry ttl expired and the dentry discarded.
> > > 
> > > A ttl (say jiffies) set at appropriate times could be a better
> > > choice all round, no sysctl values at all.
> > 
> > The canonical argument in favour of negative dentries is to improve
> > application startup time as every application searches the library
> > path
> > for the same libraries.  Only they don't do that any more:
> > 
> > $ strace -e file cat /dev/null
> > execve("/bin/cat", ["cat", "/dev/null"], 0x7ffd5f7ddda8 /* 44 vars
> > */) = 0
> > access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file
> > or
> > directory)
> > openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
> > openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6",
> > O_RDONLY|O_CLOEXEC) = 3
> > openat(AT_FDCWD, "/usr/lib/locale/locale-archive",
> > O_RDONLY|O_CLOEXEC) = 3
> > openat(AT_FDCWD, "/dev/null", O_RDONLY) = 3
> > 
> > So, are they still useful?  Or should we, say, keep at most 100
> > around?
> 
> Who knows how old apps will be on distros., ;)

Or what admins put in the PATH, I've seen oddness in that
a lot.

> 
> But I don't think it matters.

And I don't think I made my answer to the question clear.

I don't think setting a minimum matters but there are other
sources of a possibly significant number of lookups on
paths that don't exist. I've seen evidence recently
(although I suspect unfounded) that systemd can generate
lots of these lookups at times.

And let's not forget that file systems are the primary
source of these and not all create them on lookups.
I may be mistaken, but I think ext4 does not while xfs
definitely does.

The more important metric I think is calculating a sensible
maximum to be pruned to prevent getting bogged down as there
could be times when a lot of these are present. After all this
is meant to be an iterative pruning measure.

Ian