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SubjectRe: [PATCH 1/8] drivers/random: Cache align ip_random better
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On Wed, 2011-03-16 at 10:17 -0700, Hugh Dickins wrote:
> On Sun, 13 Mar 2011, George Spelvin wrote:
>
> > Cache aligning the secret[] buffer makes copying from it infinitesimally
> > more efficient.
> > ---
> > drivers/char/random.c | 2 +-
> > 1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/drivers/char/random.c b/drivers/char/random.c
> > index 72a4fcb..4bcc4f2 100644
> > --- a/drivers/char/random.c
> > +++ b/drivers/char/random.c
> > @@ -1417,8 +1417,8 @@ static __u32 twothirdsMD4Transform(__u32 const buf[4], __u32 const in[12])
> > #define HASH_MASK ((1 << HASH_BITS) - 1)
> >
> > static struct keydata {
> > - __u32 count; /* already shifted to the final position */
> > __u32 secret[12];
> > + __u32 count; /* already shifted to the final position */
> > } ____cacheline_aligned ip_keydata[2];
> >
> > static unsigned int ip_cnt;
>
> I'm intrigued: please educate me. On what architectures does cache-
> aligning a 48-byte buffer (previously offset by 4 bytes) speed up
> copying from it, and why? Does the copying involve 8-byte or 16-byte
> instructions that benefit from that alignment, rather than cacheline
> alignment?

I think this alignment exists to minimize the number of cacheline
bounces on SMP as this can be a pretty hot structure in the network
stack. It could probably benefit from a per-cpu treatment.

--
Mathematics is the supreme nostalgia of our time.




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