netdev - Re: tcp_bbr: Forcing set of BBR congestion control as default

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Date:   Mon, 2 Jan 2017 19:49:02 +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 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?

Which tc version is required?
Here tc is from iproute (20121211-2~precise).
Is that enough?

> 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

> Hope that helps,

Thanks you reply helped for further testings.

- Sedat -

P.S.: Note2myself: Enable NET_SCH_FQ

$ ./scripts/diffconfig /boot/config-4.9.0-2-iniza-amd64 .config
 NET_SCH_FQ n -> y