Subject: Re: net_sched: precision problem of TBF/HTB

From: Ding Tianhong <[email-masked]>
Date: Wed, 17 Jul 2013 09:41:03 +0800

On 2013/7/16 12:50, Eric Dumazet wrote:
> On Tue, 2013-07-16 at 12:12 +0800, Yang Yingliang wrote:
>> Hi, Eric
>> Commit 1def9238d4aa2 (net_sched: more precise pkt_len computation)
>> makes more precise transfer bytes by taking account of headers in
>> qdisc_skb_cb(skb)->pkt_len, but this introduces a problem with
>> calculating bandwidth in userland.
>>
> This changed nothing to userland.
> 
>> When calculating bandwidth in userland, it's not include headers'
>> bytes. From the user's perspective, it's not a correct bandwidth.
>>
> 
> 
>> Shall we need take account of headers in qdisc_skb_cb(skb)->pkt_len or
>> just skb->len?
>>
> These values are not accessible from userland, unless you capture
> packets with a sniffer (tcpdump or something like that)
> 
>> Example:
>> tc qdisc add dev $DEV root handle 1: tbf latency 50ms burst 500kB rate
>> 500mbit mtu 64k
>>
>> iperf -c host -t 30 -i 10
>> [ ID] Interval       Transfer     Bandwidth
>> [  3]  0.0-10.0 sec   571 MBytes   479 Mbits/sec
>> [  3] 10.0-20.0 sec   570 MBytes   478 Mbits/sec
>> [  3] 20.0-30.0 sec   570 MBytes   478 Mbits/sec
>> [  3]  0.0-30.0 sec  1.67 GBytes   478 Mbits/sec
> 
> 
> iperf only measures the amount of payload, and probably do not care of
> headers.
> 
> You cannot accurately measure bandwidth from userland, unless making
> assumptions (or getting them from the stack) on header sizes (IP + TCP),
> MSS value, and retransmits.
> 
> 

ok, for the further, can you give me some advise for how calculate the bandwidth for net link.
for example,what kind of tools is better.

> 
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