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

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Date:	Thu, 6 Mar 2014 13:09:59 -0500
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	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?

Eric's recent "auto corking" feature may be helpful in this context:

  http://lwn.net/Articles/576263/

neal

On Thu, Mar 6, 2014 at 12:17 PM, Rick Jones <rick.jones2@...com> wrote:
> 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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