netdev - Re: UDP is bypassing qdisc statistics ....

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Date:	Tue, 1 Sep 2009 19:07:57 +0930
From:	Mark Smith <lk-netdev@...netdev.nosense.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jarek Poplawski <jarkao2@...il.com>,
	Mark Smith <lk-netdev@...netdev.nosense.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	netdev@...r.kernel.org, davem@...ux-foundation.org
Subject: Re: UDP is bypassing qdisc statistics ....

Hi Eric,

On Tue, 01 Sep 2009 09:00:46 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Jarek Poplawski a écrit :
> > On 31-08-2009 22:58, Mark Smith wrote:
> >> On Mon, 31 Aug 2009 21:54:49 +0200
> >> Eric Dumazet <eric.dumazet@...il.com> wrote:
> >>
> >>> Christoph Lameter a écrit :
> >>>> This is with 2.6.31-rc7. If I send icmp then its correctly registered as a
> >>>> packet by the qdisc layer:
> >>>>
> >> <snip>
> >>> loopback device do bypass qdisc layer for example...
> >> On occassion, I'd have found it useful if it didn't. It'd be convenient
> >> to test out your qdisc config, or test out applications performance
> >> behaviour over a simulated WAN via netem, without having to a
> >> network and two hosts, and all the related miscellaneous setup work.
> > 
> > Probably Eric and you mean something special, but generally a loopback
> > and some other virtuals bypass qdisc layer only with default qdisc.
> > 
> 
> Yes, I was referring to Christoph use, since on its machine, only
> output from "tc -s -d qdisc" is about eth0
> 
> 
> Mark, you can certainly do something like
> 
> # tc qdisc del dev lo root
> # tc qdisc add dev lo root netem delay 100ms 10ms
> 
> # ping 127.0.0.1
> PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
> 2009/08/01 08:59:22.799 64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=204 ms
> 2009/08/01 08:59:23.804 64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=208 ms
> 2009/08/01 08:59:24.801 64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=204 ms
> 2009/08/01 08:59:25.808 64 bytes from 127.0.0.1: icmp_seq=4 ttl=64 time=209 ms
> 
> # tc -s -d qdisc show dev lo
> qdisc netem 8001: root limit 1000 delay 100.0ms  10.0ms
>  Sent 1764 bytes 18 pkt (dropped 0, overlimits 0 requeues 0)
>  rate 0bit 0pps backlog 0b 0p requeues 0

Either I tested it a couple of years back and you couldn't assign a
qdisc to loopback, and/or I read somewhere that the loopback driver was
highly optimised for looping packets back, and assumed that meant that
it bypassed the qdisc layer. 

Oh well, that's great that it already does what I was asking for.

Thanks,
Mark.
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