linux-kernel - Re: [PATCH 00/13] VFS: Filesystem information [ver #19]

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Date:   Wed, 01 Apr 2020 16:51:43 +0100
From:   David Howells <dhowells@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     dhowells@...hat.com, Ian Kent <raven@...maw.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux NFS list <linux-nfs@...r.kernel.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Anna Schumaker <anna.schumaker@...app.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Linux API <linux-api@...r.kernel.org>,
        linux-ext4@...r.kernel.org,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Karel Zak <kzak@...hat.com>, Jeff Layton <jlayton@...hat.com>,
        linux-fsdevel@...r.kernel.org,
        LSM <linux-security-module@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/13] VFS: Filesystem information [ver #19]

Miklos Szeredi <miklos@...redi.hu> wrote:

> For   30000 mounts, f=    146400us f2=    136766us p=   1406569us p2=
>   221669us; p=9.6*f p=10.3*f2 p=6.3*p2

	f =    146400us
	f2=    136766us
	p =   1406569us  <--- Order of magnitude slower
	p2=    221669us

And more memory used because it's added a whole bunch of inodes and dentries
to the cache.  For each mount that's a pair for each dir and a pair for each
file within the dir.  So for the two files my test is reading, for 30000
mounts, that's 90000 dentries and 90000 inodes in mountfs alone.

	(gdb) p sizeof(struct dentry)
	$1 = 216
	(gdb) p sizeof(struct inode)
	$2 = 696
	(gdb) p (216*696)*30000*3/1024/1024
	$3 = 615

so 615 MiB of RAM added to the caches in an extreme case.

We're seeing customers with 10000+ mounts - that would be 205 MiB, just to
read two values from each mount.

I presume you're not going through /proc/fdinfo each time as that would add
another d+i - for >1GiB added to the caches for 30000 mounts.

David