Re: Problem with HTB bandwidth slicing when using TCP traffic

[Andy Furniss <adf.lists@gmail.com>]

Slavica Tomovic wrote:
> Hi to all,
>
> I am using CentOS 6.4 and have problems when I want to limit
> bandwidth of TCP flow to some value smaller than 15 Mbit/s. Namely, I
> used iperf to generate TCP traffic and limited bandwidth (with tc
> command) for that flow on  6 Mbit/s. I got approximately 6 Mbit/s in
> average but iperf, which I adjusted to report me statistics every
> second, showed that in one second flow got 10 Mbit/s or more and than
> for few consecutive seconds 0 Mbit/s. With UDP traffic everything
> works fine. I expected that TCP bandwidth will fluctuate because of
> congestion mechanism but not like this. When I reserve more than 15
> Mbit/s situation is pretty much OK.
>
> Also I had similar problem when I tried to split link bandwidth
> (which I had throttled previously on 10 Mbit/s with tc) between two
> TCP flows. On the other side, TCPvsUDP and UDPvsUDP slicing works
> fine.
>
> I had updated kernel version to 2.6.32-431 recently. I don't know
> did this cause a problem because I didn't use tc htb mechanism in
> the older version.
>
> Do you have any idea why is this happening and how I can fix it?
>
> These are commands I had used to create htb classes:
>
> tc class add dev eth0 parent 1: classid 1:1 htb rate 10000kbps ceil
> 10000kbps tc class add dev eth0 parent 1:1 classid 1:10 htb rate
> 4000kbps ceil 4000kbps tc class add dev eth0 parent 1:1 classid 1:11
> htb rate 6000kbps ceil 6000kbps

kbps means k bytes/sec, use kbit or mbit

If you don't specify child qdiscs for htb it will use pfifo with the
txqlen on the device as the limit which may be a bit long on eth (1000)
or on ppp too short (3).