[LARTC] r2q error with HTB

[LARTC] r2q error with HTB

[Patrick Turley]

(This is a re-statement of a question I asked earlier)

I have a number of HTB classes feeding into a root HTB qdisc. Whenever I
set the rate on any of the subordinate classes to 78 kpbs or less, I get
the following message:


HTB : quantum of class <class ID> is small. Consider r2q change.


I've done some reading about the meaning of r2q, and I understand it
now: 

    quantum = rate/r2q

where:

    rate is expressed in kilobits per second
    quantum is expressed in bytes
    r2q has the appropriate units and is 10 by default

Based on what I've read, it's not at all clear why HTB would complain
about a rate of 78Kbit. That corresponds to a quantum of 7800 bytes,
which is much larger than the maximum Ethernet packet size (1500).

Anyone have a clue?


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Re: [LARTC] r2q error with HTB

[Stef Coene]

On Friday 08 August 2003 00:20, Patrick Turley wrote:
> (This is a re-statement of a question I asked earlier)
>
> I have a number of HTB classes feeding into a root HTB qdisc. Whenever I
> set the rate on any of the subordinate classes to 78 kpbs or less, I get
> the following message:
>
>
> HTB : quantum of class <class ID> is small. Consider r2q change.
>
>
> I've done some reading about the meaning of r2q, and I understand it
> now:
>
>     quantum = rate/r2q
>
> where:
>
>     rate is expressed in kilobits per second
>     quantum is expressed in bytes
>     r2q has the appropriate units and is 10 by default
>
> Based on what I've read, it's not at all clear why HTB would complain
> about a rate of 78Kbit. That corresponds to a quantum of 7800 bytes,
> which is much larger than the maximum Ethernet packet size (1500).
>
> Anyone have a clue?
Yes.  78kbit = 9.75kbyte.  So quantum = 9.75kilobyte/10 = 975byte.

Stef

-- 

stef.coene@docum.org
 "Using Linux as bandwidth manager"
     http://www.docum.org/
     #lartc @ irc.oftc.net

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Re: [LARTC] r2q error with HTB

[Patrick Turley]

On Fri, 2003-08-08 at 00:51, Stef Coene wrote:
> On Friday 08 August 2003 00:20, Patrick Turley wrote:
> > (This is a re-statement of a question I asked earlier)
> >
> > I have a number of HTB classes feeding into a root HTB qdisc. Whenever I
> > set the rate on any of the subordinate classes to 78 kpbs or less, I get
> > the following message:
> >
> >
> > HTB : quantum of class <class ID> is small. Consider r2q change.
> >
> >
> > I've done some reading about the meaning of r2q, and I understand it
> > now:
> >
> >     quantum = rate/r2q
> >
> > where:
> >
> >     rate is expressed in kilobits per second
> >     quantum is expressed in bytes
> >     r2q has the appropriate units and is 10 by default
> >
> > Based on what I've read, it's not at all clear why HTB would complain
> > about a rate of 78Kbit. That corresponds to a quantum of 7800 bytes,
> > which is much larger than the maximum Ethernet packet size (1500).
> >
> > Anyone have a clue?
> Yes.  78kbit = 9.75kbyte.  So quantum = 9.75kilobyte/10 = 975byte.
> 
> Stef

<blush>

OK, I wish I hadn't made that silly mistake.

79Kbit = 80896 bits/sec = 10112 bytes/sec

10112 bytes/sec / 10 r2q => quantum > 1000 bytes

I conclude, then, that HTB will spit out a warning when the quantum
implied by the subclass drops below 1000 bytes. This is useful
information.

As I said before, I have a number of classes feeding into the root.
These classes can have widely varying rate limits, and they can change
dynamically. Therefore, to avoid encountering the problem that this
message indicates, I need to estimate the lowest reasonable rate limit
and select an r2q that will just barely keep this rate's quantum above
1000. A lower rate limit of 50Kbit seems reasonable for my application:

50Kbit = 51200 bits/sec = 6400 bytes/sec

6400 bytes/sec / 1000 bytes = 6 r2q

According to Stef's FAQ at
http://qos.dyndns.org:3389/cgi-bin/fom?_highlightWords=r2q&file1, the
buit-in limit for the quantum is 60000 bytes. Even though I've reduced
the value of r2q, I still won't hit this limit until the rate exceeds:

60000 bytes * 6 r2q = 360000 bytes/sec => 2.74Mbit

This is acceptable.


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