On 05/15/2012 11:09 PM, Eric Dumazet wrote:
On Wed, 2012-05-16 at 08:55 +0300, Jonathan Morton wrote:
On 9 May, 2012, at 4:04 pm, Kevin Gross wrote:
From the paper (figure 7), you can see that CoDel still leaves spikes of
buffer occupancy when network conditions change. These will still be disruptive
to real-time traffic. Many networks that need QoS now will still need QoS.
Networks that do not have QoS will be much more usable with CoDel.
Combining AQM with FQ certainly seems like a good idea to me. I haven't had a
chance to try the implementation of fq_codel which already exists yet, but it's
compiled and just waiting for me to get around to it. If it works, then it
should be an excellent default.
fq_codel has no priority concepts. It's only Fair. But what is
Fairness ?
In the presence of elephant flows, you get more packets drops from
elephant flows than from thin ones.
The overall effects of fq_codel's implementation on top of codel's idea
of good queue vs bad queue
has yet to be evaluated. I'd like to see people explore iw10 and TCP
mice, for example on an apache benchmark emulating a typical web site,
as well as typical accesses of a web site, as well as TCP_RR stats
against saturating forward and reverse traffic, with and without ecn, at
rates ranging from 1Mbit to 40GigE.
I'm hoping to get some of that said myself, but it's a lot to do, so I
hope more people jump on it.
Then there's wireless.
All that said, I am loving what I see of fq_codel so far. I think in
general fq is superior to anything a human fiddling with priorities can do.
Also I have hooked up qfq to codel in the debloat script, which might
provide some useful comparisons to fq_codel, and also offers the
possibility of using qfq to hash on device, and fq_codel to fq within
flows to a given device, thus yielding 1/d behavior.
But I am throwing ENOTIME, ENOBRAINCELLS, and ENOENERGY errors a lot
personally, right now. I'd planned to be on vacation last week and
really need some time in a yurt far away from anything electronic.
I am looking forward very much towards published results, analysis, and
benchmarks from everyone else. Certainly I intend to get to where I can
do CDF plots of what I reference below over large
sample periods, etc, at some point... but for your low latency pleasure:
50 netperf -l 120 -t TCP_STREAM, then 1 netperf -l 30 -t TCP_RR, ecn on,
100Mbit ethernet, 3 hops
1 machine has codel, the other does not (so many kernels, so little time)
I note that QFQ is WAY more cpu/memory overhead and I haven't captured
or analysed the streams.
TCP_RR PING
PFIFO_FAST on both, no load: 2440.42 .26 to .59
PFIFO_FAST to PFIFO_FAST: 8.13 121ms
CODEL to PFIFO_FAST: 27.56 Initial impulse of
117ms, settles down to about 25ms
FQ_CODEL to PFIFO_FAST: 272.76 1.2ms, no spike
QFQ+CODEL to PFIFO_FAST: 1002.39 .7ish, no spike
QFQ+FQ_CODEL to PFIFO_FAST 1020.62 .95ms no spike (we are down at
statistical noise levels)
I haven't tried codel to codel or fq_codel to fq_codel much yet.
I note also that I get best results when also applying fq_codel as an
ingress qdisc. With that
on it's hard to get any latencies in excess of 5ms in any direction.
Without it, traffic in the incoming direction (via TCP_MAERTS on
netperf) cracks 30ms worth of delay and 110 TCP_RR vs pfifo_fast.
in debloat is script that does that using a three tier pfifo_fast
superset. Using ifb is rocket science, as is tc, however. It would be
very interesting to come up with a simpler way of using codel or
fq_codel on ingress, perhaps with tbf.
I just checked in the latest revision of debloat and the simple_qos and ingres
scripts to the deBloat repository
on github.
>If the elephant flow is the 'high priority' traffic, dropping packets
from it is not what you wanted.
pfifo_fast is able to have strict priority.
Which can lead to starvation of all flows, in pfifo_fast's case. Two
possible compromises are
using a 3 tier htb bucket to emulate pfifo_fasts behavior with the
addition of codel for sane
packet drop, or to wedge some interesting behavior into the extra 15
bits in the codel vars.
If you want Codel and strict
priority, you can use prio + codel (or prio + fq_codel)
I'm not fond of strict prioritization.
I am unsure of how useful mq_prio is at this point.
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