netdev - Re: inaccurate packet scheduling

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Message-ID: <20130207161327.GA1619@minipsycho.brq.redhat.com>
Date:	Thu, 7 Feb 2013 17:13:27 +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

Wed, Jan 30, 2013 at 01:14:47PM CET, jiri@...nulli.us wrote:
>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 :/

Any other ideas?
							
>
>Thanks
>
>Jiri
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