[Bloat] Initial tests with BBR in kernel 4.9
Eric Dumazet
eric.dumazet at gmail.com
Wed Jan 25 18:33:21 EST 2017
On Thu, 2017-01-26 at 00:04 +0100, Hans-Kristian Bakke wrote:
> I can do that. I guess I should do the capture from tun1 as that is
> the place that the tcp-traffic is visible? My non-virtual nic is only
> seeing OpenVPN encapsulated UDP-traffic.
>
But is FQ installed at the point TCP sockets are ?
You should give us "tc -s qdisc show xxx" so that we can check if
pacing (throttling) actually happens.
> On 25 January 2017 at 23:48, Neal Cardwell <ncardwell at google.com>
> wrote:
> On Wed, Jan 25, 2017 at 5:38 PM, Hans-Kristian Bakke
> <hkbakke at gmail.com> wrote:
> Actually.. the 1-4 mbit/s results with fq sporadically
> appears again as I keep testing but it is most likely
> caused by all the unknowns between me an my
> testserver. But still, changing to pfifo_qdisc seems
> to normalize the throughput again with BBR, could this
> be one of those times where BBR and pacing actually is
> getting hurt for playing nice in some very variable
> bottleneck on the way?
>
>
> Possibly. Would you be able to take a tcpdump trace of each
> trial (headers only would be ideal), and post on a web site
> somewhere a pcap trace for one of the slow trials?
>
>
> For example:
>
>
> tcpdump -n -w /tmp/out.pcap -s 120 -i eth0 -c 1000000 &
>
>
>
> thanks,
> neal
>
>
>
>
> On 25 January 2017 at 23:01, Neal Cardwell
> <ncardwell at google.com> wrote:
> On Wed, Jan 25, 2017 at 3:54 PM, Hans-Kristian
> Bakke <hkbakke at gmail.com> wrote:
> Hi
>
>
> Kernel 4.9 finally landed in Debian
> testing so I could finally test BBR in
> a real life environment that I have
> struggled with getting any kind of
> performance out of.
>
>
> The challenge at hand is UDP based
> OpenVPN through europe at around 35 ms
> rtt to my VPN-provider with plenty of
> available bandwith available in both
> ends and everything completely unknown
> in between. After tuning the
> UDP-buffers up to make room for my 500
> mbit/s symmetrical bandwith at 35 ms
> the download part seemed to work
> nicely at an unreliable 150 to 300
> mbit/s, while the upload was stuck at
> 30 to 60 mbit/s.
>
>
> Just by activating BBR the bandwith
> instantly shot up to around 150 mbit/s
> using a fat tcp test to a public
> iperf3 server located near my VPN exit
> point in the Netherlands. Replace BBR
> with qubic again and the performance
> is once again all over the place
> ranging from very bad to bad, but
> never better than 1/3 of BBRs "steady
> state". In other words "instant WIN!"
>
>
> Glad to hear it. Thanks for the test report!
>
> However, seeing the requirement of fq
> and pacing for BBR and noticing that I
> am running pfifo_fast within a VM with
> virtio NIC on a Proxmox VE host with
> fq_codel on all physical interfaces, I
> was surprised to see that it worked so
> well.
> I then replaced pfifo_fast with fq and
> the performance went right down to
> only 1-4 mbit/s from around 150
> mbit/s. Removing the fq again regained
> the performance at once.
>
>
> I have got some questions to you guys
> that know a lot more than me about
> these things:
> 1. Do fq (and fq_codel) even work
> reliably in a VM? What is the best
> choice for default qdisc to use in a
> VM in general?
>
>
> Eric covered this one. We are not aware of
> specific issues with fq in VM environments.
> And we have tested that fq works sufficiently
> well on Google Cloud VMs.
>
> 2. Why do BBR immediately "fix" all my
> issues with upload through that
> "unreliable" big BDP link with
> pfifo_fast when fq pacing is a
> requirement?
>
>
> For BBR, pacing is part of the design in order
> to make BBR more "gentle" in terms of the rate
> at which it sends, in order to put less
> pressure on buffers and keep packet loss
> lower. This is particularly important when a
> BBR flow is restarting from idle. In this case
> BBR starts with a full cwnd, and it counts on
> pacing to pace out the packets at the
> estimated bandwidth, so that the queue can
> stay relatively short and yet the pipe can be
> filled immediately.
>
>
> Running BBR without pacing makes BBR more
> aggressive, particularly in restarting from
> idle, but also in the steady state, where BBR
> tries to use pacing to keep the queue short.
>
>
> For bulk transfer tests with one flow, running
> BBR without pacing will likely cause higher
> queues and loss rates at the bottleneck, which
> may negatively impact other traffic sharing
> that bottleneck.
>
> 3. Could fq_codel on the physical host
> be the reason that it still works?
>
>
> Nope, fq_codel does not implement pacing.
>
> 4. Do BBR _only_ work with fq pacing
> or could fq_codel be used as a
> replacement?
>
>
> Nope, BBR needs pacing to work correctly, and
> currently fq is the only Linux qdisc that
> implements pacing.
>
> 5. Is BBR perhaps modified to do the
> right thing without having to change
> the qdisc in the current kernel 4.9?
>
>
> Nope. Linux 4.9 contains the initial public
> release of BBR from September 2016. And there
> have been no code changes since then (just
> expanded comments).
>
>
> Thanks for the test report!
>
>
> neal
>
>
>
>
>
>
>
>
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