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

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B6EEF@saturn3.aculab.com>
Date:	Tue, 24 Apr 2012 09:44:47 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric Dumazet" <eric.dumazet@...il.com>,
	"David Miller" <davem@...emloft.net>
Cc:	<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 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.

	David


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