From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <shemminger@osdl.org>
Subject: Re: [PATCH] convert hh_lock to seqlock
Date: Thu, 7 Dec 2006 13:15:53 -0800
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Cc: David Miller <davem@davemloft.net>, netdev@vger.kernel.org
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To: Eric Dumazet <dada1@cosmosbay.com>
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On Thu, 07 Dec 2006 21:23:07 +0100
Eric Dumazet <dada1@cosmosbay.com> wrote:

> Stephen Hemminger a =C3=A9crit :
> > The hard header cache is in the main output path, so using
> > seqlock instead of reader/writer lock should reduce overhead.
> >=20
>=20
> Nice work Stephen, I am very interested.
>=20
> Did you benchmarked it ?
>=20
> I ask because I think hh_refcnt frequent changes may defeat the gain =
you want=20
> (ie avoiding cache line ping pongs between cpus). seqlock are definit=
ly better=20
> than rwlock, but if we really keep cache lines shared.
>=20
> So I would suggest reordering fields of hh_cache and adding one=20
> ____cacheline_aligned_in_smp to keep hh_refcnt in another cache line.
>=20
> (hh_len, hh_lock and hh_data should be placed on a 'mostly read' cach=
e line)
>=20
> Thank you
> Eric

It doesn't make any visible performance difference for real networks;=20
copies and device issues are much larger.

The hh_refcnt is used only when creating destroying neighbor entries,
so except under DoS attack it doesn't make a lot of difference.
The hh_lock is used on each packet sent.