[Cerowrt-devel] [Codel] better mixing in fq_codel

Dave Taht dave.taht at gmail.com
Thu Aug 30 19:19:17 EDT 2012

On Thu, Aug 30, 2012 at 4:06 PM, Eric Dumazet <eric.dumazet at gmail.com> wrote:
> On Thu, 2012-08-30 at 15:59 -0700, Dave Taht wrote:
>> I have finally found the source of the issues I was having with htb +
>> fq_codel at low bandwidths, and it wasn't htb to the extent I thought
>> it was.
>> It was fq_codel's use of byte quantums, which was resulting in head of
>> line blocking for multiple streams.
>> The undesirable behavior (quantum of 1500)
>> A new stream, 15 acks (1500 bytes or so)
>> B new stream, 4 acks (66 bytes each)
>> C new stream, 1 packet, 1500 bytes
>> Each stream would be delivered in an entire quantum's quantity before
>> the next. This is basically something that is nearly unnoticable at
>> higher bandwidths, but down here in the slow moving mud, it's quite
>> significant. At 1 mbit, 1500 bytes is forever...
>> Better mixing behavior results from
>> A 1 packet
>> B 1 packet
>> C 1 packet (if not larger than quantum)
>> A 1 packet ""
>> B 1 packet ""
>> ...
>> While I have a patch for this (all 5 lines of it), which does this
>> sort of mixing, it crashes under load, but I'll get there. Behavior
>> before it crashes is rather nice, where before I was observing
>> something like 36 ms of delay with htb for: small packets, "sparse" or
>> ANT streams under a variety of loads, it drops below 3ms in the
>> general case for those. Both qos-scripts and simple-qos benefit
>> hugely.
>> One of these three sets of changes to fq_codel_dequeue or enqueue is
>> dubious. (well, I have a half dozen other patches and fixups to codel
>> and fq_codel in the queue too, so have to rip out each). But yea! this
>> is the low bandwidth behavior we want!
> You dont need a patch, fq_codel has a quantum parameter, default to 1514
> Just set it to 256 if you really feel the need.

In that case it will deliver 3 acks in a row from
stream A, and then 3 acks in stream B, in the linux 3.5 version, and
push the the 1500 byte packet from my example to the old flows queue -
instead of delivering A 1 ack, B 1 ack, push the C 1500 byte packet to
the old flows queue, then deliver A,B,A,B,A,B, A, A, A etc (and note
with more flows mixing gets better) as I just proposed.

(actually I got the idea from the current ns2 sfqcodel model.)

This variant however preserves the "prioritize the new stream slightly
idea", admittedly less than before...


Dave Täht
http://www.bufferbloat.net/projects/cerowrt/wiki - "3.3.8-17 is out
with fq_codel!"

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