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Re: CBQ vs. TBF



On Sun, 27 Feb 2000, jamal wrote:

> > 	With correct bandwidth, weight, rate, maxburst, avpkt etc
> > things seem to go correctly.  (maxburst out of sync with iface rate
> > and wrong avpkt screws the calculations)
> > 
> > 	Another helpful item when doing small allocs (16Kbit -
> > 256Kbit) on fast links (10 - 100 FD ethernet) is cranking the HZ value
> > from 100 on X86 to 1024.  IIRC the QOS stuff does its control every
> > HZ, by default, and every 10ms is not very often on a high rate
> > interface so small allocs tend to go over.  HZ=1024 is every ~ 1ms
> > (just under) instead of every 10ms.  I find this useful when I am
> > doing service based controls, ie having say 1:4 being a 512Kbit
> > class with different services going into 1:4 with a policed u32
> > filter.
> > 
> 
> A rule of thumb is as follows:
> max achievable rate = size * maxburst * 8 * Hz
> 
> for a maxburst of 20; size of 1000 bytes; Hz 100 (10ms timer)
> achievable rate =   1000*20*8*100 which equals 16Mbps;
> increasing Hz to 1000 makes it 160Mbps


> You can also up the maxburst; but what will happen is that you'll most
> likely start seeing on-off burst as CBQ penalizes the flow for exceeding
> its bandwith and on to allow it to send after some timeout.
> Infact as you keep increasing maxburst you should see that you get bursts
> of data going out at wire speed, then penalization by CBQ, then another
> burst at wire speed etc
> This should be a good exercise. Any volunteers?


	Already done.  When playing with low bit rate flows I kept
watching traffic levels bounce, wreaking havoc with transfers until I
got maxburst taken care of.  The machine was bursting 5 - 10 seconds
of traffic at a time over a 100Mbit iface.


> Increasing Hz to 1Khz is probably Ok for machines >= pentium pros.

	Works well, most of our Linux routers and BW managers are K6-3
450Mhz w 1MB L2 or Athlon 500Mhz - 700Mhz

> Increasing the packet size in the estimate will result in an
> over-estimation if small packet sizes are the average seen. Vice-versa if
> large packets are seen.

	Exactly.

> Note that in Linux however, the accuracy of the bandwidth measurement can
> be improved by using a different clock source.

	Yes, this is the reason I crank HZ up, at very low and very
high flow rates 10ms does not cut it.

---
As folks might have suspected, not much survives except roaches, 
and they don't carry large enough packets fast enough...
        --About the Internet and nuclear war.