Re: TCP Success Story (was Re: TCP_RACK, TCP_BBR, and firewalls)

From: <tuexen_at_freebsd.org>
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
>> 
>