linux-kernel - Re: [PATCH 00/11] fs/dcache: Limit # of negative dentries

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Date:   Fri, 28 Feb 2020 12:52:52 +0800
From:   Ian Kent <raven@...maw.net>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        Waiman Long <longman@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        Eric Sandeen <sandeen@...hat.com>
Subject: Re: [PATCH 00/11] fs/dcache: Limit # of negative dentries

On Fri, 2020-02-28 at 04:22 +0000, Al Viro wrote:
> 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.

Ok, but my thoughts aren't based on autofs behaviours.

But it sounds like you don't believe this is a sensible suggestion
and you would know best so ...

Ian