netdev - Re: inaccurate packet scheduling

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Date:	Thu, 24 Jan 2013 06:01:27 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jiri Pirko <jiri@...nulli.us>
Cc:	edumazet@...gle.com, netdev@...r.kernel.org, kuznet@....inr.ac.ru,
	jhs@...atatu.com
Subject: Re: inaccurate packet scheduling

On Thu, 2013-01-24 at 09:05 +0100, Jiri Pirko wrote:
> Tue, Jan 08, 2013 at 02:30:38PM CET, jiri@...nulli.us wrote:
> >Wed, Jan 02, 2013 at 05:34:42PM CET, eric.dumazet@...il.com wrote:
> >>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 
> >>
> >
> >To my untrained eye, 2) or 4) look like a way to go. Not sure though.
> 
> What do you think would be the best way Eric? Thanks.

Capping gso_max_size is a bit difficult after commit
1def9238d4aa2146924 (net_sched: more precise pkt_len computation)

If MSS is really small, we can easily get a lot of overhead

qdisc_skb_cb(skb)->pkt_len > 2 * skb->len

It looks like we need 4), maybe using same mechanism
than the one used in commit 56b765b79e9a78d
(htb: improved accuracy at high rates)




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