netdev - Re: [PATCH net-next] tcp: auto corking

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Date:	Fri, 06 Dec 2013 09:46:16 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] tcp: auto corking

On Fri, 2013-12-06 at 08:06 -0800, Rick Jones wrote:
> On 12/06/2013 02:30 AM, David Laight wrote:
> >> From: Eric Dumazet <edumazet@...gle.com>
> >>
> >> With the introduction of TCP Small Queues, TSO auto sizing, and TCP
> >> pacing, we can implement Automatic Corking in the kernel, to help
> >> applications doing small write()/sendmsg() to TCP sockets.
> >
> > Presumably this has the greatest effect on connections with Nagle
> > disabled?
> 
> I was wondering why Nagle didn't catch these things as well.  The 
> netperf command line Eric provided though didn't include the 
> test-specific -D option that would have disabled Nagle.  At least not 
> unless the "super_netperf" wrapper was adding it.
> 
> So, why doesn't Nagle catch what is presumably a sub-MSS send while 
> there is data outstanding on the connection?

super_netperf do not add any option.

Note the netperf results do no really show the improvements of this
patch. It's a side effect of the short cut.

You kind of need a TCP_RR workload, but splitting the request into small
chunks.

Well written applications use TCP_CORK or MSG_MORE, but unfortunately
many applications are not well written.

(Or people tried TCP_CORK in the past and got bitten by various bugs in
TCP stack that we fixed only last year)


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