netdev - Re: inaccurate packet scheduling

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Date:	Wed, 02 Jan 2013 08:34:42 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jiri Pirko <jiri@...nulli.us>
Cc:	edumazet@...gle.com, netdev@...r.kernel.org
Subject: Re: inaccurate packet scheduling

On Wed, 2013-01-02 at 16:26 +0100, Jiri Pirko wrote:
> Tue, Dec 18, 2012 at 05:51:43PM CET, erdnetdev@...il.com wrote:
> >On Tue, 2012-12-18 at 08:26 -0800, Stephen Hemminger wrote:
> >
> >> Check kernel log for messages about clock. It could be that on the
> >> machines with issues TSC is not usable for kernel clock.
> >> Also turn off TSO since it screws up any form of rate control.
> >
> >Yes, but we should fix it eventually. I'll take a look.
> 
> Hi Eric, did you have a chance to look at this? Or should I give it a try?

I took a look, and TBF does :

if (qdisc_pkt_len(skb) > q->max_size)
    return qdisc_reshape_fail(skb, sch);

We have several choices :

1) Add a one time warning

2) cap dev->gso_max_size at Qdisc setup time

3) Re-segment the packet instead of dropping it (if GSO packet), and
call the expensive qdisc_tree_decrease_qlen() function.

4) remove max_size limitation 



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