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

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Date:	Wed, 16 Jan 2013 12:22:43 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Cong Wang" <amwang@...hat.com>, <netdev@...r.kernel.org>
Cc:	"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

> 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.

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.

	David

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