Messages in this thread | | | Date | Mon, 27 Nov 2006 22:52:04 +0100 | From | Eric Dumazet <> | Subject | Re: [PATCH] fs : reorder some 'struct inode' fields to speedup i_size manipulations |
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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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