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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
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