[LARTC] SFQ latency

[LARTC] SFQ latency

[Tomas Simonaitis]

Anyone notices that using sfq qdisc adds latency compared to pfifo?

With "sfq perturb 10" ping latency is around 100-120ms, while with "pfifo 
limit 3" drops to ~10-30ms (qdiscs belong to htb class).

I'm missing something or is this expected?

{
$TC_CLASS parent 1:1 classid 1:20 htb prio 0 rate 900kbit ceil 1000kbit burst 
7k
$TC_QDISC parent 1:20 handle 200: {pfifo limit 3} | {sfq perturb 10}
}
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Re: [LARTC] SFQ latency

[Andy Furniss]

Tomas Simonaitis wrote:
> Anyone notices that using sfq qdisc adds latency compared to pfifo?
> 
> With "sfq perturb 10" ping latency is around 100-120ms, while with "pfifo 
> limit 3" drops to ~10-30ms (qdiscs belong to htb class).
> 
> I'm missing something or is this expected?
> 
> {
> $TC_CLASS parent 1:1 classid 1:20 htb prio 0 rate 900kbit ceil 1000kbit burst 
> 7k
> $TC_QDISC parent 1:20 handle 200: {pfifo limit 3} | {sfq perturb 10}
> }

There shouldn't really be a queue as such (you still choose one) for 
your interactive traffic - if you really care about latency HTB should 
have a class with a high (low number) prio and a high rate (> expected 
traffic), so other traffic has to borrow.

If you do end up queueing your interractive, then pfifo limit 3 means 
there will only be 2 packets in front of the ping (I suppose bfifo would 
be nicer as a fall back - it will penalise big packets more, WRT drops). 
SFQ default qlen is 128, though the way it works means the delay will 
depend on how many other connections are queued.


Andy.


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Re: [LARTC] SFQ latency

[Tomas Simonaitis]

Thanks Andy.

{P.S. 1:20 is class for interactive traffic, i skipped other}
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