Andrew Morton a écrit :
On Thu, 23 Nov 2006 11:57:29 +0100
Eric Dumazet <[email protected]> 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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