From: John Fastabend <john.fastabend@gmail.com>
To: Ding Tianhong <dingtianhong@huawei.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
Yang Yingliang <yangyingliang@huawei.com>,
Eric Dumazet <edumazet@google.com>,
netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>
Subject: Re: net_sched: precision problem of TBF/HTB
Date: Tue, 16 Jul 2013 20:18:15 -0700 [thread overview]
Message-ID: <51E60CF7.8020500@gmail.com> (raw)
In-Reply-To: <51E5F62F.8080906@huawei.com>
On 07/16/2013 06:41 PM, Ding Tianhong wrote:
> 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.
>
>>
Vendor specific but it looks reasonably common to count tx and rx bytes
in ethtool stats. Take a few samples with 'ethtool -S' and do the
math. This will give you what the driver/hardware actually xmits and
receives.
.John
--
John Fastabend Intel Corporation
prev parent reply other threads:[~2013-07-17 3:19 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-07-16 4:12 net_sched: precision problem of TBF/HTB Yang Yingliang
2013-07-16 4:50 ` Eric Dumazet
2013-07-17 1:41 ` Ding Tianhong
2013-07-17 3:18 ` John Fastabend [this message]
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