Re: [LARTC] HTB and PRIO qdiscs introducing extra latency when output

[Andy Furniss <andy.furniss@dsl.pipex.com>]

Jonathan Lynch wrote:

> Could anyone tell me why the delay is so high (30ms) for VoIP packets
> which are treated with the EF phb when the outgoing interface of core
> router to network c is saturated ?
> 

I have never used dsmark so am not sure about the classification parts 
of your rules. You need to check where the packets are going with with 
tc -s qdisc ls dev ...

The other parts have some issues see below.

> -----------------------------------------------------------------------------------------------
> The config I used for each setup is included below. These are slight
> modifications that are supplied with iproute2 source code.
> 
> Config 1 using htb
> -------------------
> tc qdisc add dev $1 handle 1:0 root dsmark indices 64 set_tc_index
> tc filter add dev $1 parent 1:0 protocol ip prio 1 tcindex mask 0xfc
> shift 2

flowid/classid here maybe, to get packets to 2:0, though it may work - 
check.

> 
> Main htb qdisc & class
> tc qdisc add dev $1 parent 1:0 handle 2:0 htb
> tc class add dev $1 parent 2:0 classid 2:1 htb rate 100Mbit ceil 100Mbit

100mbit will be too high if it's 100mbit nic.

> 
> EF Class (2:10)
> tc class add dev $1 parent 2:1 classid 2:10 htb rate 1500Kbit ceil
> 100Mbit
> tc qdisc add dev $1 parent 2:10 pfifo limit 5
> tc filter add dev $1 parent 2:0 protocol ip prio 1 handle 0x2e tcindex
> classid 2:10 pass_on

Don't know what pass_on will mean here.

> 
> BE Class (2:20)
> tc class add dev $1 parent 2:1 classid 2:20 htb rate 5Mbit ceil 100Mbit
> tc qdisc add dev $1 parent 2:20 red limit 60KB min 15KB max 45KB burst
> 20 avpkt 1000 bandwidth 100Mbit probability 0.4
> tc filter add dev $1 parent 2:0 protocol ip prio 2 handle 0 tcindex mask
> 0 classid 2:20 pass_on
> 
> Config 2 using PRIO
> -------------------
> Main dsmark & classifier
> tc qdisc add dev $1 handle 1:0 root dsmark indices 64 set_tc_index
> tc filter add dev $1 parent 1:0 protocol ip prio 1 tcindex mask 0xfc
> shift 2
> 
> Main prio queue
> tc qdisc add dev $1 parent 1:0 handle 2:0 prio
> tc qdisc add dev $1 parent 2:1 tbf rate 1.5Mbit burst 1.5kB limit 1.6kB

Won't hurt if the packets are small voip but TBF has a nasty habit of 
taking 1 from the burst/mtu you specify so your burst setting may result 
in packets >1499B getting dropped - tc -s -d qdisc ls dev ... should 
show what it's using.

> tc filter add dev $1 parent 2:0 protocol ip prio 1 handle 0x2e tcindex
> classid 2:1 pass_on
> 
> BE class(2:2)
> tc qdisc add dev $1 parent 2:2 red limit 60KB min 15KB max 45KB burst 20
> avpkt 1000 bandwidth 100Mbit probability 0.4
> tc filter add dev $1 parent 2:0 protocol ip prio 2 handle 0 tcindex mask
> 0 classid 2:2 pass_on

Without wrapping it with something like htb red won't shape traffic.

Andy.
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