netdev - Re: inaccurate packet scheduling

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Message-ID: <20130130121447.GB1604@minipsycho.orion>
Date:	Wed, 30 Jan 2013 13:14:47 +0100
From:	Jiri Pirko <jiri@...nulli.us>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	edumazet@...gle.com, netdev@...r.kernel.org, kuznet@....inr.ac.ru,
	jhs@...atatu.com
Subject: Re: inaccurate packet scheduling

Tue, Jan 29, 2013 at 05:13:03PM CET, eric.dumazet@...il.com wrote:
>On Tue, 2013-01-29 at 13:23 +0100, Jiri Pirko wrote:
>
>> part of the commit message says:
>> <quote>
>> The bits per second on the wire is still 5200Mb/s with new HTB
>> because qdisc accounts for packet length using skb->len, which
>> is smaller than total bytes on the wire if GSO is used.  But
>> that is for another patch regardless of how time is accounted.	
>> </quote>
>> I believe that is a similar problem like ours. But looks like this
>> "another patch" never got in.
>> 
>
>Hmm, I thought I addressed this in 
>
>commit 1def9238d4aa2146924994aa4b7dc861f03b9362
>Author: Eric Dumazet <edumazet@...gle.com>
>Date:   Thu Jan 10 12:36:42 2013 +0000
>
>    net_sched: more precise pkt_len computation
>    
>    One long standing problem with TSO/GSO/GRO packets is that skb->len
>    doesn't represent a precise amount of bytes on wire.
>    
>    Headers are only accounted for the first segment.
>    For TCP, thats typically 66 bytes per 1448 bytes segment missing,
>    an error of 4.5 % for normal MSS value.
>    
>    As consequences :
>    
>    1) TBF/CBQ/HTB/NETEM/... can send more bytes than the assigned limits.
>    2) Device stats are slightly under estimated as well.
>    
>    Fix this by taking account of headers in qdisc_skb_cb(skb)->pkt_len
>    computation.
>    
>    Packet schedulers should use qdisc pkt_len instead of skb->len for their
>    bandwidth limitations, and TSO enabled devices drivers could use pkt_len
>    if their statistics are not hardware assisted, and if they don't scratch
>    skb->cb[] first word.
>    
>    Both egress and ingress paths work, thanks to commit fda55eca5a
>    (net: introduce skb_transport_header_was_set()) : If GRO built
>    a GSO packet, it also set the transport header for us.
>


I tried kernel with this patch in. I also ported
56b765b79e9a78dc7d3f8850ba5e5567205a3ecd to tbf. I'm getting always the
similar numbers with iperf. There must be something else needed :/

Thanks

Jiri
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