> On 18. Jul 2024, at 15:00, Junho Choi <junho.c...@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 <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 outdated, but that's still pretty close for such a simple formula 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
