Re: net_sched: precision problem of TBF/HTB

On 07/16/2013 06:41 PM, Ding Tianhong wrote:

On 2013/7/16 12:50, Eric Dumazet wrote:

quoted

On Tue, 2013-07-16 at 12:12 +0800, Yang Yingliang wrote:

quoted

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.

quoted

When calculating bandwidth in userland, it's not include headers'
bytes. From the user's perspective, it's not a correct bandwidth.

quoted

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)

quoted

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.

quoted

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