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

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Date:	Thu, 06 Mar 2014 06:38:11 -0800
From:	Eric Dumazet <eric.dumazet@...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?

On Thu, 2014-03-06 at 12:28 +0000, 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.
> 
> 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.)

This is the intent of TCP small queue, but it was somehow broken by
latest patch.

Also try FQ pacing

tc qdisc replace dev eth0 root fq

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 5286228679bd..6e1db9889a8f 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1909,8 +1909,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 		 * of queued bytes to ensure line rate.
 		 * One example is wifi aggregation (802.11 AMPDU)
 		 */
-		limit = max_t(unsigned int, sysctl_tcp_limit_output_bytes,
-			      sk->sk_pacing_rate >> 10);
+		limit = 2 * skb->truesize;
 
 		if (atomic_read(&sk->sk_wmem_alloc) > limit) {
 			set_bit(TSQ_THROTTLED, &tp->tsq_flags);


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