Messages in this thread | | | From | Oleksandr Natalenko <> | Subject | Re: TCP and BBR: reproducibly low cwnd and bandwidth | Date | Fri, 16 Feb 2018 18:35:03 +0100 |
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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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