netdev - Re: Can I limit the number of active tx per TCP socket?

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Message-ID: <5318ADB1.5050107@hp.com>
Date:	Thu, 06 Mar 2014 09:17:37 -0800
From:	Rick Jones <rick.jones2@...com>
To:	David Laight <David.Laight@...LAB.COM>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Can I limit the number of active tx per TCP socket?

On 03/06/2014 04:28 AM, David Laight wrote:
> Is it possible to stop a TCP connection having more than one
> tx skb (in the ethernet tx ring) at any one time?
> The idea is to allow time for short sends from the application
> to accumulate so that the transmitted frames are longer.

That is precisely what Nagle is supposed to be doing - at least where 
the definition of "time" is the round-trip-time rather than "time it 
takes to get transmitted out the NIC."

> Basically I have a TCP connection which carries a lot of separate
> short 'user buffers'. These are not command-response so
> TCP_NODELAY has to be set to avoid long delays.

When you are saturating the receiver and/or the 64K line, are you 
certain that not setting TCP_NODELAY means long delays?

 From a later message:

> The data is sent out on a 64k line so 1ms is only 8 byte times.

Are you still using a 1460 byte MSS on such a connection?

Perhaps you can set the MSS (or drop the MTU on the 64K line and use 
PTMU) to something less to trigger window updates a bit sooner and so 
get piggy-backed ACKs rather than delayed ACKs and so not have to set 
TCP_NODELAY?  Yes, you will have a question of headers versus 
headers+data but with TCP_NODELAY set as you have it you are (probably) 
already trashing that.

Setting TCP_NODELAY to avoid "long delays" and then having a 64Kbyte/s 
link seems a trifle, well, contradictory.

rick jones
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