Re: [PATCH] fs : reorder some 'struct inode' fields to speedup i_size manipulations

[Andrew Morton <akpm@osdl.org>]

On Thu, 23 Nov 2006 11:57:29 +0100
Eric Dumazet <dada1@cosmosbay.com> wrote:

> On 32bits SMP platforms, 64bits i_size is protected by a seqcount 
> (i_size_seqcount).
> 
> When i_size is read or written, i_size_seqcount is read/written as well, so it 
> make sense to group these two fields together in the same cache line.
> 
> Before this patch, accessing i_size needed 3 cache lines (2 for i_size, one 
> for i_size_seqcount). After, only one cache line is needed/ (dirtied on a 
> i_size change).

I didn't understand that paragraph at all, really, so I took it out.

At present an i_size change will dirty one, two or three cachelines, most
likely one or two.

After your patch an i_size change will dirty one or two cachelines, most
likely one.

yes?

> This patch moves i_size_seqcount next to i_size, and also moves i_version to 
> let offsetof(struct inode, i_size) being 0x40 instead of 0x3c (for 32bits 
> platforms). 
> 
> For 64 bits platforms, i_size_seqcount doesnt exist, and the move of a 'long 
> i_version' should not introduce a new hole because of padding.
> 
> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>