Date:   Mon, 2 Jan 2017 20:30:58 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     Netdev <netdev@...r.kernel.org>
Subject: Re: tcp_bbr: Forcing set of BBR congestion control as default

On Mon, Jan 2, 2017 at 8:12 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
> On Mon, Jan 2, 2017 at 1:49 PM, Sedat Dilek <sedat.dilek@...il.com> wrote:
>> On Mon, Jan 2, 2017 at 7:17 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
>>> On Mon, Jan 2, 2017 at 12:05 AM, Sedat Dilek <sedat.dilek@...il.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> I am trying to force the set of BBR congestion control as default.
>>>> My old linux-config uses CUBIC as default.
>>>> I want both BBR and CUBIC to be built but BBR shall be my default.
>>>>
>>>> I tried the below snippet.
>>>>
>>>> I refresh my new linux-config like this...
>>>>
>>>> $ MAKE="make V=1" ; COMPILER="mycompiler" ; MAKE_OPTS="CC=$COMPILER
>>>> HOSTCC=$COMPILER" ; yes "" | $MAKE $MAKE_OPTS oldconfig && $MAKE
>>>> $MAKE_OPTS silentoldconfig < /dev/null
>>>>
>>>> What am I doing wrong?
>>>
>>> Perhaps your build directory already has an old .config file that sets
>>> the DEFAULT_TCP_CONG to be "cubic", and your "make oldconfig" and
>>> "make silentoldconfig" are maintaining those lines from the old
>>> .config?
>>>
>>> If you want to start with your existing .config but then make BBR the
>>> default, then it might be simplest just to edit your .config directly:
>>>
>>> -CONFIG_DEFAULT_TCP_CONG="cubic"
>>> +CONFIG_DEFAULT_TCP_CONG="bbr"
>>>
>>
>> Just to clarify...
>>
>> I can have both TCP_CONG cubic and bbr built and switch for example via sysctl?
>
> Yes, you can set the default congestion control using sysctl, eg:
>
>   sysctl net.ipv4.tcp_congestion_control=bbr
>
> And (as mentioned in the quick-start guide) on many Linux systems you
> can make this "sticky" after reboots with something like:
>
>   sudo bash -c 'echo "net.ipv4.tcp_congestion_control=bbr" >> /etc/sysctl.conf'
>
>> Which tc version is required?
>> Here tc is from iproute (20121211-2~precise).
>> Is that enough?
>
> You should not need a particular version of tc to install "fq".
>
> In the BBR quick-start guide we recommend setting fq to be the default qdisc:
>
>   sudo bash -c 'echo "net.core.default_qdisc=fq" >> /etc/sysctl.conf'
>

OK, this looks now good.

# sysctl net.core.default_qdisc
net.core.default_qdisc = fq

# sysctl net.ipv4.tcp_congestion_control
net.ipv4.tcp_congestion_control = bbr

I wondered why my Internet connection has so many stalls.

> But if that doesn't work in your environment, then I believe you
> should be able to install the fq qdisc with any version of tc, with
> something like:
>
>   tc qdisc replace dev eth0 root fq
>
> You just won't be able to set or view configuration options.
>
>>> BTW, I presume you have seen the warning in the BBR commit message or
>>> tcp_bbr.c about ensuring that BBR is used with the "fq" qdisc:
>>>
>>>   NOTE: BBR *must* be used with the fq qdisc ("man tc-fq") with pacing
>>>   enabled, since pacing is integral to the BBR design and
>>>   implementation. BBR without pacing would not function properly, and
>>>   may incur unnecessary high packet loss rates.
>>>
>>> The BBR quick-start guide has some details about how to build and
>>> enable BBR and fq:
>>>
>>>   https://github.com/google/bbr/blob/master/Documentation/bbr-quick-start.md
>>>
>>
>> Hmmm...
>>
>> From [1] Section "Further reading"...
>>
>> egrep '(CONFIG_TCP_CONG_BBR|CONFIG_NET_SCH_FQ)=' .config
>>
>> then you see exactly the following lines:
>>
>> CONFIG_TCP_CONG_BBR=y
>> CONFIG_NET_SCH_FQ=y
>>
>> Should CONFIG_TCP_CONG_BBR have a "select CONFIG_NET_SCH_FQ" in its Kconfig?
>> That would be safer.
>>
>> [1] https://github.com/google/bbr/blob/master/Documentation/bbr-quick-start.md
>
> That would be a little safer, but not sufficient (since the qdisc
> still has to be configured to be in the transmit path somewhere).
>

Does BBR only work with fq-qdisc best?
What about fq_codel?

- Sedat -

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