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Message-ID: <34fccefda12dce92706bc40c25626846616de264.camel@kernel.org>
Subject: Re: [PATCH] fs: switch timespec64 fields in inode to discrete
 integers
From: Jeff Layton <jlayton@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Christian Brauner <brauner@kernel.org>, Alexander Viro
 <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.cz>, Linus Torvalds
 <torvalds@linux-foundation.org>, linux-fsdevel@vger.kernel.org, Amir
 Goldstein <amir73il@gmail.com>, linux-kernel@vger.kernel.org,
 dhowells@redhat.com
Date: Sat, 18 May 2024 06:48:30 -0400
In-Reply-To: <Zkg7OCSYJ7rzv6_D@casper.infradead.org>
References: <20240517-amtime-v1-1-7b804ca4be8f@kernel.org>
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On Sat, 2024-05-18 at 06:23 +0100, Matthew Wilcox wrote:
> On Fri, May 17, 2024 at 08:08:40PM -0400, Jeff Layton wrote:
> > For reference (according to pahole):
> >=20
> >     Before:	/* size: 624, cachelines: 10, members: 53 */
> >     After: 	/* size: 616, cachelines: 10, members: 56 */
>=20
> Smaller is always better, but for a meaningful improvement, we'd want
> to see more.  On my laptop running a Debian 6.6.15 kernel, I see:
>=20
> inode_cache        11398  11475    640   25    4 : tunables    0    0    =
0 : slabdata    459    459      0
>=20
> so there's 25 inodes per 4 pages.  Going down to 632 is still 25 per 4
> pages.  At 628 bytes, we get 26 per 4 pages.  Ar 604 bytes, we're at 27.
> And at 584 bytes, we get 28.
>=20
> Of course, struct inode gets embedded in a lot of filesystem inodes.
> xfs_inode         142562 142720   1024   32    8 : tunables    0    0    =
0 : slabdata   4460   4460      0
> ext4_inode_cache      81     81   1184   27    8 : tunables    0    0    =
0 : slabdata      3      3      0
> sock_inode_cache    2123   2223    832   39    8 : tunables    0    0    =
0 : slabdata     57     57      0
>=20
> So any of them might cross a magic boundary where we suddenly get more
> objects per slab.
>=20
> Not trying to diss the work you've done here, just pointing out the
> limits for anyone who's trying to do something similar.  Or maybe
> inspire someone to do more reductions ;-)

Way to bust my bubble, Willy. ;-)

On a more serious note, I may be able to squeeze out another 4 bytes by
moving __i_ctime to a single 8 byte word. It's never settable from
userland, so we probably don't need the full range of times that a
timespec64 gives us there. Shrinking that may also make the multigrain
time rework simpler.

David Howells was also looking at removing the i_private field as well.
Since these structs are usually embedded in a larger structure, it's
not clear that we need that field. If we can make that work, it'll mean
another 8 bytes goes away on 64-bit arches.

IOW, I think there may be some other opportunities for shrinkage in the
future.
--=20
Jeff Layton <jlayton@kernel.org>