[Cake] cake target corner cases?
moeller0 at gmx.de
Mon Nov 2 06:29:42 EST 2015
On Nov 1, 2015, at 21:58 , Alan Jenkins <alan.christopher.jenkins at gmail.com> wrote:
> On 01/11/2015, Sebastian Moeller <moeller0 at gmx.de> wrote:
>> Dear cake committee,
>> I just played around with the most recent sch_cake and noticed:
>> user at computer:~/CODE/tc-adv/tc> sudo tc-adv qdisc del dev eth0 root
>> user at computer:~/CODE/tc-adv/tc> sudo tc-adv qdisc replace dev eth0 root cake
>> bandwidth 1Mbit ; sudo tc-adv -s qdisc
>> qdisc cake 8005: dev eth0 root refcnt 6 bandwidth 1Mbit diffserv4 flows rtt
>> 100.0ms raw
>> Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
>> backlog 0b 0p requeues 0
>> capacity estimate: 1Mbit
>> Tin 0 Tin 1 Tin 2 Tin 3
>> thresh 1Mbit 937504bit 750Kbit 250Kbit
>> target 18.2ms 19.4ms 24.2ms 72.7ms
>> interval 145.3ms 155.0ms 193.8ms 581.4ms
>> Here target is always 12.5% of interval instead of the expected 6.25%
>> 1/16 = 0.0625
>> 72.7/581.4 = 0.125042999656
>> 24.2/193.8 = 0.124871001032
>> 19.4/155.0 = 0.125161290323
>> 18.2/145.3 = 0.125258086717
>> But the bandwidth is really low, so pushing target closer to the bandwidth
>> conserving side of the codel rationale might be fine,
> Pretty sure it's a minimum derived from the MTU
> ((mtu=1.5kbyte) * 8 bits/byte) / 1000 Mbit/s = 0.012s
> except I don't know where the .5 comes from, that's incredibly
> suspicious to have a round 1/8th :).
As we agree later/earlier 1/8th it is ;)
> The point is that if buffering falls below the MTU, the connection
> will be completely clobbered.
> In a way it's nice cake reports this in the target. Otherwise cake
> would claim the target is 5ms, but measurements would show the
> effective target is more than twice as high.
>> since latency is bad
>> to begin with and bandwidth also pretty scarce. But it might be interesting
>> to do a few more measurements at low bandwidths to confirm that the 12.5% of
>> interval logic holds water; one could also argue that people with such links
>> (a lot of DSL lines have even less upload, so this certainly is not extreme)
>> might think that any added ms of delay matters (more than bandwidth);
>> currently we leave the user no remedy...
>> This looks okay, except Tin3 has target at 7.3/101.0 = 0.0722772277228 7% of
> Looks like the same thing.
Well, I believe there is a gradual shift from the default 5ms/6.2ms target to the 1/8th target and this is just somewhere in between, by virtue of the smallest bandwidth or so...
>> Both observations might actually be on purpose, but if so we should document
>> that behavior as expected, for example in the man page…
>> Best Regards
> I'm afraid I can't help mention my old niggle :). _If_ you mention
> this alongside instructions for RRUL, I think you'd also want to
> explain^W mention the measurement increase for diffserv4 v.s.
> I think the ICMP ping measurement increases by another 10ms on my
> connection (11500k down / 850k up, so an mtu is ~15ms). I concluded
> it was inherent in prioritization. Now I guess it's equal to the sum
> of target * bandwidth_fraction for each class "above" icmp ping (and
> could be tested).
Well, with standard sqm-scripts (htb and fq_codel) we leverage iptables to do the filtering, which comes with its own computation cost… then again with your bandwidths there should be enough cycles left to do this...
> I have graphs from sqm with and without classification. I did test
> cake once and I think it's the same (otherwise would be a bug).
Yes, that is the picture I know, I believe cake looks a bit different, by virtue of doing a few things in a more integrated, clever way; not sure how it looks at low bandwidth though, I rarely test at low bandwidth nowadays. Even though I should…
> Warm regards
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