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Date:   Fri, 28 Feb 2020 04:22:08 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Ian Kent <raven@themaw.net>, Andreas Dilger <adilger@dilger.ca>,
        Waiman Long <longman@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
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        Eric Biggers <ebiggers@google.com>,
        Dave Chinner <david@fromorbit.com>,
        Eric Sandeen <sandeen@redhat.com>
Subject: Re: [PATCH 00/11] fs/dcache: Limit # of negative dentries
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On Thu, Feb 27, 2020 at 07:34:12PM -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:

	Tell that to scripts that keep looking through $PATH for
binaries each time they are run.  Tell that to cc(1) looking through
include path, etc.

	Ian, autofs is deeply pathological in that respect; that's OK,
since it has very unusual needs, but please don't use it as a model
for anything else - its needs *are* unusual.