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From: Ewa Janczukowicz <janczukowicz.ewa@gmail.com>
To: lartc@vger.kernel.org
Subject: 
Date: Tue, 12 Apr 2016 13:12:20 +0000	[thread overview]
Message-ID: <CAPTuX4pQdmcJHt22ht_ZRcz+Nx=f9FYWvFB9hUWfGuUwYn+xcA@mail.gmail.com> (raw)

Hello,

I would like to ask a question about a weird (at least for me J) HTB
behavior that I get, while prioritizing one type of traffic.

I am working on assuring low delay for UDP traffic at the home gateway
level. At this home gateway I have two types of traffic, TCP and UDP,
and I assure differentiated treatment by using HTB.
The bandwidth I am testing equals 1Mbit/s.

 Thus, I have to types of leaf classes:
- UDP leaf class with:
- the highest priority,

- a short queue length (SFQ qdisc),

- assured rate 200kbit/s and ceil rate 1Mbit/s,

- quantum = 3 x MTU.

- TCP leaf class with

- lowest priority,

- default queue length (pFIFO qdisc),

- minimum assured rate (8bit/s – to force it to stay in yellow mode
most of the time)  and ceil rate 1Mbit/s,

- quantum = MTU.



In order to see how the traffic interacts, for UDP I have a stairs
type of traffic, thus I start at 0bit/s and I increase the traffic
every ten seconds by 100kbit/s. When I reach 1Mbit/s I decrease every
10s by 100kbit/s until I reach zero.

Alongside, I have TCP traffic, either a file upload, either a simple
iperf (without any influence on observed behavior).



Normally, most of the time, I get an expected behavior. Thus I can see
perfectly the traffic separation and the “stairs” trend of the UDP.
Additionally, UDP traffic takes over TCP (but TCP can still send – and
the trend is the opposite of UDP, thus first decreasing, later
increasing).



However when the UDP bitrate is already decreasing (about 30 seconds
before reaching 0), TCP traffic completely takes over for a couple of
seconds. I can’t really understand this behavior, because it seems
that UDP traffic cannot send, but it shouldn’t be in “red” mode since
its bitrate is already decreasing.

I think it has something to do with HTB scheduling and blocking UDP
traffic for some reason.



I hope my question is clear, but I can also provide wireshark bitrate graphs.

I will continue to test different configurations but I will appreciate
any suggestions.



Thank you in advance for your help.

Ewa

                 reply	other threads:[~2016-04-12 13:12 UTC|newest]

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