Re: [LARTC] icmp latency question

[Sebastian Bork <sebi@sebi.org>]

Andrew Beverley wrote:
> It may have been a stupid question, but it does raise an issue I have 
> noticed. Even if I give maximum priority to SSH/ICMP, I notice that the 
> latency does still jump around. Is this because I've not set up the 
> queueing discipline very well?

Did you shorten the queue on the network interface? If there are already
some packets which the kernel dequeued to the interface, your ICMP
packet will have to wait until they have been sent. The only longer
queue should be the one in the kernel, because only there the high
priority traffic gets queued before the already waiting packets, all
hardware queues are FIFOs.

 > For SSH I have:
 > tc qdisc add dev imq0 parent 1:10 handle 10: sfq perturb 10

You should not use sfq for high priority traffic, it is only useful for 
bulk traffic - there should never be as much high priority traffic as to 
make sfq needed to stop one flow from starving another, and if you use 
it, your latency will rise.

-- 
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