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

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1CAD027E@AcuExch.aculab.com>
Date:	Wed, 21 Jan 2015 12:26:36 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Rick Jones' <rick.jones2@...com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Dave Taht <dave.taht@...il.com>
CC:	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"

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.

	David