Re: [PATCH 1/2] xfs: reorder xfs_inode structure elements to remove unneeded padding.

[JunChao Sun <sunjunchao2870@gmail.com>]

Matthew Wilcox <willy@infradead.org> 于2024年6月19日周三 00:51写道：
>
> On Tue, Jun 18, 2024 at 12:40:23PM -0400, JunChao Sun wrote:
> > Darrick J. Wong <djwong@kernel.org> 于2024年6月18日周二 12:23写道：
> > >
> > > On Tue, Jun 18, 2024 at 07:35:04PM +0800, Junchao Sun wrote:
> > > > By reordering the elements in the xfs_inode structure, we can
> > > > reduce the padding needed on an x86_64 system by 8 bytes.
> > >
> > >
> > > > Does this result in denser packing of xfs_inode objects in the slab
> > > > page?
> >
> > No. Before applying the patch, the size of xfs_inode is 1800 bytes
> > with my config, and after applying the patch, the size is 1792 bytes.
> > This slight reduction does not result in a denser packing of xfs_inode
> > objects within a single page.
>
>
> > The "config dependent" part of this is important though.  On my
> > laptop running Debian 6.6.15-amd64, xfs_inode is exactly 1024 bytes,
> > and slab chooses to allocate 32 of them from an order-3 slab.
> >
> > Your config gets you 18 from an order-3 slab, and you'd need to get
> > it down to 1724 (probably 1720 bytes due to alignment) to get 19
> > from an order-3 slab.  I bet you have lockdep or something on.

Hi,

I couldn't find the exact 6.6.15-amd64 kernel, but I installed the
Debian 6.8.12-amd64 and 6.1.0-21-amd64 kernels, along with their
corresponding debug packages. In both cases, the size of xfs_inode is
1000 bytes. By eliminating the padding bytes, the number of xfs_inode
objects allocated from an order-3 slab increased from 32 to 33.

I'm not sure what specific differences there are between our Debian
kernels, but I have submitted the v2 version of the patch. If there
are any issues, please feel free to let me know. Thank you!


Best regards
-- 
Junchao Sun <sunjunchao2870@gmail.com>