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From: Eric Dumazet <eric.dumazet@gmail.com>
To: Juliusz Chroboczek <jch@pps.univ-paris-diderot.fr>
Date: Tue, 28 Jul 2015 16:18:42 +0200
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Cc: bloat <bloat@lists.bufferbloat.net>
Subject: Re: [Bloat] AQM and PPP on Linux
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On Tue, 2015-07-28 at 15:50 +0200, Juliusz Chroboczek wrote:
> > What distro are you on?
> 
> Debian.  (One machine stable, one testing.)
> 
> > There's a sysctl to set the default qdisc: net.core.default_qdisc.
> 
>     $ sysctl -a | grep default_qdisc
>     net.core.default_qdisc = pfifo_fast
> 
> > Set this to fq_codel and you should be good to go.
> 
> Is that safe?  I'm thinking lo and virtual interfaces.

Unless some user space script change it, virtual interfaces have
txqueuelen = 0, so no qdisc is set up on them

$ ifconfig lo | grep txqueuelen
          collisions:0 txqueuelen:0

$ tc -s -d qdisc sh dev lo
<empty>

So yes, this is reasonably safe to change net.core.default_qdisc