netdev - Re: VLAN I/F's and TX queue.

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Date:	Mon, 10 May 2010 16:50:20 +0200
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Subject: Re: VLAN I/F's and TX queue.

Patrick McHardy <kaber@...sh.net> wrote on 2010/05/10 16:33:00:
>
> Joakim Tjernlund wrote:
> > Eric Dumazet <eric.dumazet@...il.com> wrote on 2010/05/07 10:53:23:
> >>> 3) I would expect lost pkgs to be accounted on eth0 instead of
> >>>    the VLAN interface(s) since that is where the pkg is lost, why
> >>>    isn't it so?
> >> You try to send packets on eth0.XXX, some are dropped, and accounted for
> >> on eth0.XXX stats. What is wrong with this ?
> >
> > In this case one lost pkg is accounted for twice, once on eth0.1 and
> > once more on eth0.1.1. Note that eth0.1.1 is stacked on
> > top of eth0.1
> >
> > I would at least expect eth0 to also account lost pkgs too.
> > I was confused by the current accounting as I knew that
> > the underlying HW I/F should be the only I/F that could
> > drop pkgs.
>
> In case of NET_XMIT_CN, the packet is dropped by the qdisc before
> it reaches eth0, so its only accounted on the upper devices.

hmm, I am afraid I don't follow this. Why would a pkg be dropped before
it reaches eth0?

>
> >> If you want to avoid this, just add queues to your vlans
> >>
> >> ip link add link eth0 eth0.103 txqueuelen 100 type vlan id 103
> >
> >>From memory now, but that didn't help. Still accounts pgks
> > as described. Why would where to account pkgs be affected by
> > queue or no queue?
>
> If a queue is used on the vlan device, it will queue the packet
> until the lower device is able to transmit it (unless its own
> queue overflows).

And if a pkg is is lost this also changes where to account dropped?
I don't understand this. The queue may prevent pkg loss to some degree
but I don't get why a queue!=0 would change on which interface to
account for lost pkg's.

      Jocke

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