netdev - Re: [PATCH net-next 3/3] tcp: better TCP_SKB_CB layout to reduce cache line misses

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Date:	Tue, 23 Sep 2014 09:36:53 +0200
From:	Christoph Paasch <christoph.paasch@...ouvain.be>
To:	Eric Dumazet <edumazet@...gle.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next 3/3] tcp: better TCP_SKB_CB layout to reduce
 cache line misses

On 23/09/14 - 09:20:16, Christoph Paasch wrote:
> Hello Eric,
> 
> On 22/09/14 - 16:50:44, Eric Dumazet wrote:
> > TCP maintains lists of skb in write queue, and in receive queues
> > (in order and out of order queues)
> > 
> > Scanning these lists both in input and output path usually requires
> > access to skb->next, TCP_SKB_CB(skb)->seq, and TCP_SKB_CB(skb)->end_seq
> > 
> > These fields are currently in two different cache lines, meaning we
> > waste lot of memory bandwidth when these queues are big and flows
> > have either packet drops or packet reorders.
> > 
> > We can move TCP_SKB_CB(skb)->header at the end of TCP_SKB_CB, because
> > this header is not used in fast path. This allows TCP to search much faster
> > in the skb lists.
> > 
> > Even with regular flows, we save one cache line miss in fast path.
> > 
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > ---
> >  include/net/tcp.h   | 12 ++++++------
> >  net/ipv4/tcp_ipv4.c |  7 +++++++
> >  net/ipv6/tcp_ipv6.c |  7 +++++++
> >  3 files changed, 20 insertions(+), 6 deletions(-)
> 
> doesn't also the tx-path relies on IPCB(skb) being memset to 0 (e.g., in
> xfrm4_output or ip_options_build)

Crap, forget about ip_options_build. It is memcpy'ing into IPCB... Confused
src with dest... :-)

Nevertheless, wouldn't there be a problem in xfrm4_output?


Cheers,
Christoph

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