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?
BR, On Thu, Jul 18, 2024 at 5:27 AM <tue...@freebsd.org> wrote: > > On 17. Jul 2024, at 22:00, Alan Somers <asom...@freebsd.org> wrote: > > > > On Sat, Jul 13, 2024 at 1:50 AM <tue...@freebsd.org> wrote: > >> > >>> On 13. Jul 2024, at 01:43, Alan Somers <asom...@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 outdated, but that's still pretty close for such a simple formula 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
