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

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Date:	Thu, 6 Mar 2014 09:15:37 -0500
From:	John Heffner <johnwheffner@...il.com>
To:	David Laight <David.Laight@...lab.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Can I limit the number of active tx per TCP socket?

Is TCP_CORK what you're looking for?


On Thu, Mar 6, 2014 at 7:28 AM, David Laight <David.Laight@...lab.com> 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.
>
> 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. But this means
> that the ethernet packet rate is very high - with 3.14 about
> 2000/sec even though the data rate is well under 1MB/sec.
>
> Anything that reduces this packet rate will help the poor little
> embedded ppc that has to receive them!
>
> From the descriptions I've found I suspect that setting a very
> low TCP_NOTSENT_LOWAT (like 1 byte) might have other side effects.
> I think that limits the writes into kernel memory - which isn't
> really what I'm trying to do.
>
> With a 3.14.0-rc5 kernel reducing the network speed to 10M (from Ge)
> halves the number of transmitted packets (with the same aggregate
> data rate). But I suspect it could still be reduced further.
>
> Limiting the number of tx packets per TCP connection might also
> help stop bulk transfers affecting low-latency connections,
> especially if the throughput of individual connections isn't
> especially important - as it may not be on a big ftp/web server.
>
> Limiting the window size offered by the remote system won't help me.
> The window needs to be large enough several full-sized packets, and
> I'm trying to stop large numbers of very short packets being sent.
>
> In this particular case the connection is local, but we have a similar
> problem with sigtran m3ua traffic over sctp.
> If we are sending 15000 sctp data chunks every second, with an average size
> under 300 bytes (possibly nearer 150) then we really want to fill the
> ethernet packets.
> (That is a real data pattern, not a development test.)
>
>         David
>
>
>
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