LARTC - Mar 2004 - Any danger in thrashing ''tc''?

Hi,

Is there any danger in a prog which repeatedly clears the ingress and 
root egress qdiscs, and sets up new ones, even as frequently as every 
5-15 seconds?

I''m writing a shaper app which dynamically shapes traffic on some weird
attributes such as:
  * country of remote peer
  * program on local machine (and/or each program''s arguments)

The objective is to apply limits to any overseas traffic to/from 
long-running p2p apps, such as I2P and Freenet, to help ADSL users in NZ 
prevent blowing their ''overseas traffic caps'' and getting hit
with nasty
bills.

It''s based on a loop which:
* does a netstat
* reads /proc/[pid] to get the dirt on each program with an active
   TCP connection
* runs a set of tests (matching for programs to traffic-shape, and how
   they should be traffic-shaped)
* derives from all this a set of ingress and root egress shaping rules
* clears the ingress and root egress qdisc, and fires off n tc commands
   to implement the shaping which is needed in the moment.

As you can see, the prog will be frequently spitting heaps of tc 
commands, constantly taking down the ingress and root egress qdiscs, and 
creating new ones.

So, am I likely to hit on any unintended consequences (apart from the 
minor cpu spikes)?

Thx in advance for your insights.

-- 

Kind regards
David

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David McNab wrote:
> Is there any danger in a prog which repeatedly clears the ingress
> and root egress qdiscs, and sets up new ones, even as frequently as
> every 5-15 seconds?
[...]
> As you can see, the prog will be frequently spitting heaps of tc
> commands, constantly taking down the ingress and root egress qdiscs,
> and creating new ones.
>
> So, am I likely to hit on any unintended consequences (apart from
> the minor cpu spikes)?
When you remove the qdiscs, you will cause all queued packets to
drop. When you add qdiscs you obviously have reseted the burst values
for all of them so then each of has a full burst to spend (if you use
bursts anywhere, that is).

This does not happen when you change parameters that can be changed by
''tc qdisc change'', but not all values can be changed. And
obviously
this does not happen if you just change iptables marking rules to mark
packets differently.

As for your actual problem - I would suggest looking for some other
solution than mangling rules/qdiscs every few seconds.

-- Naked


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