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

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Date:	Thu, 6 Mar 2014 14:09:38 -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?

On Thu, Mar 6, 2014 at 2:06 PM, Rick Jones <rick.jones2@...com> wrote:
> On 03/06/2014 10:09 AM, Neal Cardwell wrote:
>>
>> Eric's recent "auto corking" feature may be helpful in this context:
>>
>>    http://lwn.net/Articles/576263/
>
>
> Doesn't that depend on the bottleneck being local to the sending side?
> Perhaps I've mis-understood David's setup, but I get the impression the
> bottleneck is not at the sending side but either in the middle or at the
> end, so tx completions will still be happening quickly.

Yes, I think auto corking works well if you are using the fq/pacing
qdisc, and otherwise may not buy much.

neal

> rick
>
>
>> 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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