LARTC - Jun 2006 - [PATCH 0/2] Runtime configuration of HTB''s HYSTERESIS option

The HTB qdisc has a compile time option, HTB_HYSTERESIS, 
that trades accuracy of traffic classification for CPU 
time.  These patches change hysteresis to be a runtime 
option under the control of "tc".

The effects of HYSTERESIS on HTB''s accuracy are significant 
(see chapter 7, section 7.3.1, pp 69-70 in Jesper Brouer''s
thesis: http://www.adsl-optimizer.dk/thesis/ ), whereas 
HTB''s CPU usage on modern machines using broadband links 
is minimal.  Currently HYSTERESIS is on by default, and 
requires a kernel re-compile to change.  Altering it to 
be a runtime option will make life easier for the bulk of 
its users.

Further documentation on the patch and its usage can be
found here:
  http://www.stuart.id.au/russell/files/tc/tc-atm


This is a combined effort of Jesper Brouer and Russell 
Stuart, to get these patches into the upstream kernel.

Let the discussion start about what we need to change to 
get this upstream?

We see this as a feature enhancement, as such hope that 
it can be queued in davem''s net-2.6.18.git tree. 

--
Regards
Russell Stuart and Jesper Brouer


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Russell Stuart wrote:
> The HTB qdisc has a compile time option, HTB_HYSTERESIS, 
> that trades accuracy of traffic classification for CPU 
> time.  These patches change hysteresis to be a runtime 
> option under the control of "tc".
> 
> The effects of HYSTERESIS on HTB''s accuracy are significant 
> (see chapter 7, section 7.3.1, pp 69-70 in Jesper Brouer''s
> thesis: http://www.adsl-optimizer.dk/thesis/ ), whereas 
> HTB''s CPU usage on modern machines using broadband links 
> is minimal.  Currently HYSTERESIS is on by default, and 
> requires a kernel re-compile to change.  Altering it to 
> be a runtime option will make life easier for the bulk of 
> its users.
At time of HTB implementation I needed to reach 100MBit speed on 
relatively slow box. The hysteresis was a way. On other side I used 
hand-made TSC based measure tool to compute exact (15%) performance 
gain. Today I''d measure it using oprofile.

When rethinking it again I''d suggest to re-measure real performance 
impact for both flat and deep class hierarchy and consider switching the 
hysteresis off by default (or even to remove the code if the gain is 
negligible). If it is the case then it is the cleanest solution IMHO.

On other side I see no problem with attached patches. Have you tested 
patched kernel with old "tc" tool ?

thanks for your effort,
Martin
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On Thu, 2006-06-15 at 11:49 +0200, Martin Devera wrote:
> At time of HTB implementation I needed to reach 100MBit speed on 
> relatively slow box. The hysteresis was a way. On other side I used 
> hand-made TSC based measure tool to compute exact (15%) performance 
> gain. Today I''d measure it using oprofile.
> 
> When rethinking it again I''d suggest to re-measure real
performance
> impact for both flat and deep class hierarchy and consider switching the 
> hysteresis off by default (or even to remove the code if the gain is 
> negligible). If it is the case then it is the cleanest solution IMHO.
I attended LCA 2006 this year.  There was a presentation by
a group in New Zealand using Debian running on a embedded
box to bring the Internet to rural communities.  Some of
these communities didn''t have power or telephone, so the
setup ran over 802.11 over distances of up to 23Km using
solar cells for power.  I don''t recall exactly, but I think
the embedded box was using a 486 equivalent.  I think they
had around 40 of these things up and going.

The point of the story is there are people out there who
use Linux on small processors, and often do imaginative 
things with them.  We would be doing them a disservice by 
ripping out the code.
> On other side I see no problem with attached patches. Have you tested 
> patched kernel with old "tc" tool ?
Yes.

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