netdev - Re: BW regression after "tcp: refine TSO autosizing"

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Message-ID: <1421859716.4832.27.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 21 Jan 2015 09:01:56 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	'Rick Jones' <rick.jones2@...com>, Dave Taht <dave.taht@...il.com>,
	Eyal Perry <eyalpe@...lanox.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Eyal Perry <eyalpe@....mellanox.co.il>,
	Or Gerlitz <gerlitz.or@...il.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Amir Vadai <amirv@...lanox.com>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>,
	Saeed Mahameed <saeedm@...lanox.com>,
	Ido Shamay <idos@...lanox.com>, Amir Ancel <amira@...lanox.com>
Subject: Re: BW regression after "tcp: refine TSO autosizing"

On Wed, 2015-01-21 at 12:26 +0000, David Laight wrote:
> From: Of Rick Jones
> > >> Are you saying that at long last, delayed acks as we knew them are
> > >> dead, dead, dead?
> > >
> > > Sorry, I can not parse what you are saying.
> > >
> > > In case you missed it, it has nothing to do with delayed ACK but GRO on
> > > receiver.
> > 
> > Dave - assuming I've interpreted Eric's comments correctly, I believe
> > the answer to your question is No.  Your desire for a world brimming
> > with ack-every-other purity has not been fulfilled :)
> > 
> > However, the engineers formerly at Mentat are probably pleased that a
> > functional near-equivalent to their ACK avoidance heuristic has ended-up
> > being implemented and tacitly accepted, albeit by the back door :)
> 
> I must recheck something I discovered a while back with more recent kernels.
> There has been a bad interaction between 'slow start' and 'delayed acks'
> when nagle is disabled on 0 RTT local links with uni-directional traffic.
> 
> 'Slow start' would refuse to send more than 4 messages until it received
> an ack (rather than 4 mss of data).
> The receiving system wouldn't send an ack until the timer expired
> (or several mss of data were received) by which time the sender could have
> a lot of data queued.
> 
> Due to the 0 RTT and bursty nature of the data 'slow start' happened
> all the time.

Following packetdrill test suggests that current kernel send up to 10
messages without having to wait for any ACK
(IW10)

// Set up production and experiment configs
`../common/defaults.sh`

// Establish a connection.
0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
0.000 bind(3, ..., ...) = 0
0.000 listen(3, 1) = 0

0.100 < S 0:0(0) win 32792 <mss 1460,nop,wscale 7>
0.100 > S. 0:0(0) ack 1 <mss 1460,nop,wscale 6>
0.110 < . 1:1(0) ack 1 win 257
0.110 accept(3, ..., ...) = 4

0.200 %{ assert tcpi_snd_cwnd == 10 }%
+0 setsockopt(4, SOL_TCP, TCP_NODELAY, [1], 4) = 0

+0.01 write(4, ..., 100) = 100
+0  > P. 1:101(100) ack 1

+0.01 write(4, ..., 100) = 100
+0  > P. 101:201(100) ack 1

+0.01 write(4, ..., 100) = 100
+0  > P. 201:301(100) ack 1

+0.01 write(4, ..., 100) = 100
+0  > P. 301:401(100) ack 1

+0.01 write(4, ..., 100) = 100
+0  > P. 401:501(100) ack 1

+0.01 write(4, ..., 100) = 100
+0  > P. 501:601(100) ack 1

+0.01 write(4, ..., 100) = 100
+0  > P. 601:701(100) ack 1

+0.01 write(4, ..., 100) = 100
+0  > P. 701:801(100) ack 1

+0.01 write(4, ..., 100) = 100
+0  > P. 801:901(100) ack 1

+0.01 write(4, ..., 100) = 100
+0  > P. 901:1001(100) ack 1




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