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I have a question about quality of service (QoS) on linux (DDWRT specifically).

Here's my setup:

  • Recent version of DD-WRT
  • 120 kbytes/s up and 2mbytes/down
  • Traffic limited to about 80% of stated speed, so about 100kbytes/sec
  • HFSC enabled but no traffic rules.

Observed behaviour:

Traffic is indeed limited to about 80kb/s. Ping at full saturation is still low.

Problem scenario:

Say User 1 is uploading to Dropbox. He is saturating the connection at about 100kb/s. Then User 2 starts uploading to Dropbox. I'd expect User 1 and User 2 to share the bandwidth about equally, ie 50kb/s per user. In practice, User 1 may get 85kb/s and User 2 only 15kb/s. This is contrary to my expectations.

What I'd like is to guarantee that all users will have a fair slice of the available bandwidth with bursting. Are my expectations out of line here? Am I missing something?

Thanks in advance!

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2 Answers 2

What you are asking is not unreasonable, however I do not think (someone correct me if I am wrong) that QoS is best suited to distribute traffic equally between clients. Instead, it is there to make sure that certain traffic (such as VOIP) goes out of the network at a higher priority than others so that they are not interrupted by data flow of other protocols (in a nut shell).

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I'm not sure I agree. I see what you mean about priority though. What happens when two different connections to the same service are made? That's what my scenario is about. Two connections prioritized equally should share bandwidth equally.. but that doesn't seem to happen. –  EightyEight Apr 28 '11 at 1:18
    
Like I said, afaik, QoS doesn't care whether traffic is distributed equally or not between devices; QoS is there to make sure higher priority traffic goes first. Maybe there is an enhanced version of QoS that will also distribute traffic equally between devices but I am not aware of any. –  emtunc May 1 '11 at 11:25

Make sure to reduce your buffers on the saturated link.

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Thank you for your link. This is a fascinating problem. This blog has a good introduction specifically related to dd-wrt and the site that you linked has a good intro as well. –  EightyEight Apr 28 '11 at 17:35

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