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Subject: Re: [PATCH 00/11] fs/dcache: Limit # of negative dentries
To:     Matthew Wilcox <willy@infradead.org>, Ian Kent <raven@themaw.net>
Cc:     Andreas Dilger <adilger@dilger.ca>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jonathan Corbet <corbet@lwn.net>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        linux-doc@vger.kernel.org,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Eric Biggers <ebiggers@google.com>,
        Dave Chinner <david@fromorbit.com>,
        Eric Sandeen <sandeen@redhat.com>
References: <20200226161404.14136-1-longman@redhat.com>
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <e8730c5e-6610-f25a-f1cc-9d4ffffe0eb5@redhat.com>
Date:   Fri, 28 Feb 2020 10:32:02 -0500
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On 2/27/20 10:34 PM, 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?
>
It is the shell that does the path search, not the command itself.

Cheers,
Longman