Re: TCP Success Story (was Re: TCP_RACK, TCP_BBR, and firewalls)
Date: Thu, 18 Jul 2024 19:23:02 UTC
> On 18. Jul 2024, at 20:37, Alan Somers <asomers@freebsd.org> wrote: > > Coexist how? Do you mean that one socket can use one and a different > socket uses the other? That makes sense. Correct. Best regards Michael > > On Thu, Jul 18, 2024 at 10:34 AM <tuexen@freebsd.org> wrote: >> >>> On 18. Jul 2024, at 15:00, Junho Choi <junho.choi@gmail.com> wrote: >>> >>> Alan - this is a great result to see. Thanks for experimenting. >>> >>> Just curious why bbr and rack don't co-exist? Those are two separate things. >>> Is it a current bug or by design? >> Technically RACK and BBR can coexist. The problem was with pf and/or LRO. >> >> But this is all fixed now in 14.1 and head. >> >> Best regards >> Michael >>> >>> BR, >>> >>> On Thu, Jul 18, 2024 at 5:27 AM <tuexen@freebsd.org> wrote: >>>> On 17. Jul 2024, at 22:00, Alan Somers <asomers@freebsd.org> wrote: >>>> >>>> On Sat, Jul 13, 2024 at 1:50 AM <tuexen@freebsd.org> wrote: >>>>> >>>>>> On 13. Jul 2024, at 01:43, Alan Somers <asomers@FreeBSD.org> wrote: >>>>>> >>>>>> I've been experimenting with RACK and BBR. In my environment, they >>>>>> can dramatically improve single-stream TCP performance, which is >>>>>> awesome. But pf interferes. I have to disable pf in order for them >>>>>> to work at all. >>>>>> >>>>>> Is this a known limitation? If not, I will experiment some more to >>>>>> determine exactly what aspect of my pf configuration is responsible. >>>>>> If so, can anybody suggest what changes would have to happen to make >>>>>> the two compatible? >>>>> A problem with same symptoms was already reported and fixed in >>>>> https://reviews.freebsd.org/D43769 >>>>> >>>>> Which version are you using? >>>>> >>>>> Best regards >>>>> Michael >>>>>> >>>>>> -Alan >>>> >>>> TLDR; tcp_rack is good, cc_chd is better, and tcp_bbr is best >>>> >>>> I want to follow up with the list to post my conclusions. Firstly >>>> tuexen@ helped me solve my problem: in FreeBSD 14.0 there is a 3-way >>>> incompatibility between (tcp_bbr || tcp_rack) && lro && pf. I can >>>> confirm that tcp_bbr works for me if I either disable LRO, disable PF, >>>> or switch to a 14.1 server. >>>> >>>> Here's the real problem: on multiple production servers, downloading >>>> large files (or ZFS send/recv streams) was slow. After ruling out >>>> many possible causes, wireshark revealed that the connection was >>>> suffering about 0.05% packet loss. I don't know the source of that >>>> packet loss, but I don't believe it to be congestion-related. Along >>>> with a 54ms RTT, that's a fatal combination for the throughput of >>>> loss-based congestion control algorithms. According to the Mathis >>>> Formula [1], I could only expect 1.1 MBps over such a connection. >>>> That's actually worse than what I saw. With default settings >>>> (cc_cubic), I averaged 5.6 MBps. Probably Mathis's assumptions are >>>> outdated, but that's still pretty close for such a simple formula >>>> that's 27 years old. >>>> >>>> So I benchmarked all available congestion control algorithms for >>>> single download streams. The results are summarized in the table >>>> below. >>>> >>>> Algo Packet Loss Rate Average Throughput >>>> vegas 0.05% 2.0 MBps >>>> newreno 0.05% 3.2 MBps >>>> cubic 0.05% 5.6 MBps >>>> hd 0.05% 8.6 MBps >>>> cdg 0.05% 13.5 MBps >>>> rack 0.04% 14 MBps >>>> htcp 0.05% 15 MBps >>>> dctcp 0.05% 15 MBps >>>> chd 0.05% 17.3 MBps >>>> bbr 0.05% 29.2 MBps >>>> cubic 10% 159 kBps >>>> chd 10% 208 kBps >>>> bbr 10% 5.7 MBps >>>> >>>> RACK seemed to achieve about the same maximum bandwidth as BBR, though >>>> it took a lot longer to get there. Also, with RACK, wireshark >>>> reported about 10x as many retransmissions as dropped packets, which >>>> is suspicious. >>>> >>>> At one point, something went haywire and packet loss briefly spiked to >>>> the neighborhood of 10%. I took advantage of the chaos to repeat my >>>> measurements. As the table shows, all algorithms sucked under those >>>> conditions, but BBR sucked impressively less than the others. >>>> >>>> Disclaimer: there was significant run-to-run variation; the presented >>>> results are averages. And I did not attempt to measure packet loss >>>> exactly for most runs; 0.05% is merely an average of a few selected >>>> runs. These measurements were taken on a production server running a >>>> real workload, which introduces noise. Soon I hope to have the >>>> opportunity to repeat the experiment on an idle server in the same >>>> environment. >>>> >>>> In conclusion, while we'd like to use BBR, we really can't until we >>>> upgrade to 14.1, which hopefully will be soon. So in the meantime >>>> we've switched all relevant servers from cubic to chd, and we'll >>>> reevaluate BBR after the upgrade. >>> Hi Alan, >>> >>> just to be clear: the version of BBR currently implemented is >>> BBR version 1, which is known to be unfair in certain scenarios. >>> Google is still working on BBR to address this problem and improve >>> it in other aspects. But there is no RFC yet and the updates haven't >>> been implemented yet in FreeBSD. >>> >>> Best regards >>> Michael >>>> >>>> [1]: https://www.slac.stanford.edu/comp/net/wan-mon/thru-vs-loss.html >>>> >>>> -Alan >>> >>> >>> >>> >>> -- >>> Junho Choi <junho dot choi at gmail.com> | https://saturnsoft.net >> >