Re: fq_codel, htb and quantum parameter...

Linux Advanced Routing and Traffic Control

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Mandi! Dave Taht
  In chel di` si favelave...

> >> The two treatments of quantum are very different.
> > Very clear, thanks.
> Took me a long while to figure it out!

...and seems to me that we need a wiki to write down all that stuff...

> We have improved this in later versions of fq_codel (cake), so it
> figures it out for you.

i'm on a debian wheezy, so kernel 3.2.0-4-686-pae and tc as of iproute2
package version 3.12.0-2, from backports.
So i think i need to wait some more time to eat the cake. ;-)

So, to try to sum your ''rule of thumb'':

 + for ASYMMETRIC lines running around at 20Mbit/s (downlink) or less,
   it is better to set 'quantum' to 300 (bytes), that is roughly 4 ipv4
   ACK to leave fq_codel do ''automatic ACK priorization''.

 + for any other case, leave at the default.

Any ''manual ACK priorization'' is anyway considered ''deprecated''
by fq_codel, that do it internally and more eefficiently.

A question: 'quantum 300' have to be set on egress, or on
ifb/ingress side? Or both?

 + considering indipendently egress and ingress (so, companion ifb
   interface), if rates is > 2.5Mbit/s, leave the default value.

 + if rate is lower, it is needed to account for one MTU, in
   milliseconds, rounding a bit higher.
   if MTU=1500 and BANDW=640Kbit/s, the MTU in milliseconds is:

	1500*8/640000 = 19ms

   so we can round it to 20ms;

A question: interval it is safe to keep as 150ms? Or it is better to
change also that accordingly? or it is unrelated, so it is better to
set with other criteria?

 + at really low rates, roughly under 1Mbit/s, probably it is better
   to keep using SFQ instead of fw_codel, that is oriented with modern,
   high rates.

Say me if i've understood well. Thanks.

dott. Marco Gaiarin				        GNUPG Key ID: 240A3D66
  Associazione ``La Nostra Famiglia''          
  Polo FVG   -   Via della Bontà, 7 - 33078   -   San Vito al Tagliamento (PN)
  marco.gaiarin(at)   t +39-0434-842711   f +39-0434-842797

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