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Date: Fri, 16 Feb 2018 18:35:03 +0100 From: Oleksandr Natalenko <oleksandr@...alenko.name> To: Holger Hoffstätte <holger@...lied-asynchrony.com> Cc: "David S. Miller" <davem@...emloft.net>, Alexey Kuznetsov <kuznet@....inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>, Soheil Hassas Yeganeh <soheil@...gle.com>, Neal Cardwell <ncardwell@...gle.com>, Yuchung Cheng <ycheng@...gle.com>, Van Jacobson <vanj@...gle.com>, Jerry Chu <hkchu@...gle.com> Subject: Re: TCP and BBR: reproducibly low cwnd and bandwidth Hi. On pátek 16. února 2018 17:26:11 CET Holger Hoffstätte wrote: > These are very odd configurations. :) > Non-preempt/100 might well be too slow, whereas PREEMPT/1000 might simply > have too much overhead. Since the pacing is based on hrtimers, should HZ matter at all? Even if so, poor 1 Gbps link shouldn't drop to below 100 Mbps, for sure. > BBR in general will run with lower cwnd than e.g. Cubic or others. > That's a feature and necessary for WAN transfers. Okay, got it. > Something seems really wrong with your setup. I get completely > expected throughput on wired 1Gb between two hosts: > /* snip */ Yes, and that's strange :/. And that's why I'm wondering what I am missing since things cannot be *that* bad. > /* snip */ > Please note that BBR was developed to address the case of WAN transfers > (or more precisely high BDP paths) which often suffer from TCP throughput > collapse due to single packet loss events. While it might "work" in other > scenarios as well, strictly speaking delay-based anything is increasingly > less likely to work when there is no meaningful notion of delay - such > as on a LAN. (yes, this is very simplified..) > > The BBR mailing list has several nice reports why the current BBR > implementation (dubbed v1) has a few - sometimes severe - problems. > These are being addressed as we speak. > > (let me know if you want some of those tech reports by email. :) Well, yes, please, why not :). > /* snip */ > I'm not sure testing the old version without builtin pacing is going to help > matters in finding the actual problem. :) > Several people have reported severe performance regressions with 4.15.x, > maybe that's related. Can you test latest 4.14.x? Observed this on v4.14 too but didn't pay much attention until realised that things look definitely wrong. > Out of curiosity, what is the expected use case for BBR here? Nothing special, just assumed it could be set as a default for both WAN and LAN usage. Regards, Oleksandr
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