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

[Eric Dumazet <dada1@cosmosbay.com>]

Andrew Morton a écrit :
> 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?

nope

Before :
---------
 offsetof(i_size) = 0x3C

i_size is 8 bytes, so i_size spans 2 cache lines (if 64 or 32 bytes cache lines)

and offsetof(i_size_seqcount) = 0x160, so a read of i_size (coupled with a 
read of seqcount) needed 3 cache lines. A change of i_size dirtied 2 or 3 
cache lines.


After :
--------
offsetof(i_size) = 0x40
offsetof(i_size_seqcount) = 0x48

One cache line 'only', reading or writing.


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