netdev - Re: inaccurate packet scheduling

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Date:	Tue, 18 Dec 2012 08:26:31 -0800 (PST)
From:	Stephen Hemminger <stephen.hemminger@...tta.com>
To:	Jiri Pirko <jiri@...nulli.us>
Cc:	jhs@...atatu.com, davem@...emloft.net, edumazet@...gle.com,
	tgraf@...g.ch, netdev@...r.kernel.org
Subject: Re: inaccurate packet scheduling



----- Original Message -----
> Hi all.
> 
> Run one of the following 2 scripts on machine A:
> 
> #!/bin/bash
> tc qdisc del dev eth0 root
> sleep 1
> tc -batch << EOF
> qdisc add dev eth0 root handle 1: prio bands 2 priomap 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0
> qdisc add dev eth0 parent 1:1 handle 10: pfifo limit 50
> qdisc add dev eth0 parent 1:2 handle 20 tbf latency 100ms rate 4mbit
> burst 2m
> filter add dev eth0 parent 1: protocol ip u32 match ip dst
> $machineB_ip flowid 1:2
> EOF
> 
> #!/bin/bash
> tc qdisc del dev eth0 root
> sleep 1
> tc -batch << EOF
> qdisc add dev eth0 root handle 1: prio bands 2 priomap 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0
> qdisc add dev eth0 parent 1:1 handle 10: pfifo limit 20
> qdisc add dev eth0 parent 1:2 handle 20: pfifo limit 20
> filter add dev eth0 parent 1: protocol ip pref 10 \
> u32 match ip dst $machineB_ip \
> flowid 1:2 \
> police rate 4Mbit burst 2m conform-exceed drop
> EOF
> 
> And run:
> [machineB ~]# iperf -s
> [machineA ~]# iperf -c machineB_ip -t 60
> 
> Expected results are: ~3.8-4.2 Mbits/s
> But actual results are: ~130-170 Kbits/s with tbf, ~70-300 Kbits/s
> with policy rate
> 
> [machineA ~]# tc -s qdisc list dev eth0
> qdisc prio 1: root refcnt 9 bands 2 priomap  0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0
>  Sent 1512384 bytes 1032 pkt (dropped 729, overlimits 0 requeues 0)
>  backlog 0b 0p requeues 0
> qdisc pfifo 10: parent 1:1 limit 50p
>  Sent 4560 bytes 32 pkt (dropped 0, overlimits 0 requeues 0)
>  backlog 0b 0p requeues 0
> qdisc tbf 20: parent 1:2 rate 4000Kbit burst 2Mb lat 100.0ms
>  Sent 1507824 bytes 1000 pkt (dropped 729, overlimits 0 requeues 0)
>  backlog 0b 0p requeues 0
> 
> 
> Tested with kernel pulled from linus's git today. This happens with
> older
> kernels as well (I tried 2.6.32-based rhel6 kernels).
> 
> This happens to me on following machines:
> HP DL360G8 (x86_64) http://people.redhat.com/jpirko/aThoo2Ei/dl380g8/
> HP DL360G3 (i686)
> IBM JS22 (ppc64) http://people.redhat.com/jpirko/aThoo2Ei/ibmjs22/
> 
> On following machines, I do not observe this issue:
> qemu kvm (x86_64)
> IBM Zseries (s390x) http://people.redhat.com/jpirko/aThoo2Ei/ibmz/
> 
> Please ask in case you need me to provide any other details.
> 
> Thanks.

Check kernel log for messages about clock. It could be that on the
machines with issues TSC is not usable for kernel clock.
Also turn off TSO since it screws up any form of rate control.
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