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Date:	Mon, 23 Apr 2012 22:37:26 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	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...

> This proposed patch allows non-dataless packets to eat more space in
> the backlog, thus the concern and slight pushback.  And from another
> perspective, having the stack process data packets which will just
> get dropped when we try to attach it to the receive queue is just
> wasted work.

We could try to coalesce ACKs before backlogging them. I'll work on
this.

Thanks


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