netdev - RE: [PATCH 2/2 net-next] tcp: sk_add_backlog() is too agressive for TCP

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Date:	Tue, 24 Apr 2012 10:53:12 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	David Miller <davem@...emloft.net>, rick.jones2@...com,
	netdev@...r.kernel.org, therbert@...gle.com, ncardwell@...gle.com,
	maze@...gle.com, ycheng@...gle.com, ilpo.jarvinen@...sinki.fi
Subject: RE: [PATCH 2/2 net-next] tcp: sk_add_backlog() is too agressive
 for TCP

On Tue, 2012-04-24 at 09:44 +0100, David Laight wrote:
> > On Mon, 2012-04-23 at 16:01 -0400, David Miller wrote:
> > 
> > > Hmmm... why don't we just acknowledge reality and special case ACKs?
> > > 
> > 
> > Yes why not.
> > 
> > 
> > > If a TCP packet is dataless we should just let it go 
> > > through no matter what and with no limits.
> > >  It is by definition transient and will not
> > > get queued up into the socket past this backlog stage.
> > > 
> > 
> > Even being transient we need a limit. Without copybreak, an 
> > ACK can cost 2048+256 bytes.
> > 
> > In my 10Gbit tests (standard netperf using 16K buffers), I've seen
> > backlogs of 300 ACK packets...
> 
> What about forcing a copybreak for acks when above the rx buffer size?
> That way you avoid the cost of the copy in teh normal case when
> the data will be freed, but avoid the memory overhead when a lot
> of acks (or rx data) is queued.

Thats noise, as the minimal truesize of an ACK packet is 512 + 256 on
x86_64

The fact that ixgbe provides 1024 + 256 could be fixed in the driver,
its a 4 lines change actually. Then you already have a minimal skb.



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