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Date:	Wed, 21 Jan 2015 12:26:36 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Rick Jones' <>,
	Eric Dumazet <>,
	Dave Taht <>
CC:	Eyal Perry <>,
	Yuchung Cheng <>,
	"Neal Cardwell" <>,
	Eyal Perry <>,
	"Or Gerlitz" <>,
	Linux Netdev List <>,
	Amir Vadai <>,
	Yevgeny Petrilin <>,
	Saeed Mahameed <>,
	Ido Shamay <>,
	"Amir Ancel" <>
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.


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