netdev - RE: [RFC Patch net-next] tcp: add a global sysctl to control TCP delayed ack

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Date:	Thu, 17 Jan 2013 17:21:46 +0800
From:	Cong Wang <amwang@...hat.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
	Rick Jones <rick.jones2@...com>,
	Stephen Hemminger <shemminger@...tta.com>,
	"David S. Miller" <davem@...emloft.net>,
	Thomas Graf <tgraf@...g.ch>
Subject: RE: [RFC Patch net-next] tcp: add a global sysctl to control TCP
 delayed ack

On Wed, 2013-01-16 at 12:22 +0000, David Laight wrote:
> > According to previous discussion, it seems there is no
> > reasonable heuristics.
> > 
> > Similar to TCP_QUICK_ACK option, but for people who can't
> > modify the source code and still wants to control
> > TCP delayed ACK behavior.
> > 
> > Makes any sense?
> 
> A sysctl is a bit of a big hammer, it probably isn't necessary
> to disable delayed acks on all connections.

You mean make this sysctl per-socket? But we don't have per-socket or
per-connection sysctl for networking, do we?

> 
> IIRC the related problems I saw were really on the sending
> side when Nagle is disabled and it is doing 'slow start'.
> 
> Globally disabling on connections that have Nagle disabled
> might be a possibility - but it is the Nagle parameter
> at the other end that matters.
> 
> Perhaps the sending side, after sending 4 small frames immediately,
> could send 1 or 2 additional full sized frames in order to
> provoke an ack (IIRC an ack is sent if there are 2 full sized
> frames of data unacked).
> 
> The other problem is that 'slow start' is restarted very
> aggressively - whenever there is no unacked data.
> If you have a very low latency connection and aren't doing
> continuous bulk transfer it is restarted for every short
> burst of transmits - effectively after every received ack.
> There really ought to have to be a moderate idle time
> before 'slow start' is restarted.
> 

These situations are not easy at all to detect.

Thanks.

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