[Cerowrt-devel] SQM in mainline openwrt, fq_codel considered for fedora default

Tom Gundersen teg at jklm.no
Thu Dec 4 11:09:21 EST 2014

On Tue, Oct 21, 2014 at 10:59 PM, Dave Taht <dave.taht at gmail.com> wrote:
> On Tue, Oct 21, 2014 at 12:51 PM, Dave Taht <dave.taht at gmail.com> wrote:
>>> http://snapon.lab.bufferbloat.net/~d/beagle_bql/bql_makes_a_difference.png
>>> You can see that BQL makes the most difference in the latency.
>> And ALSO that these fixes also improved system throughput enormously.
> Meant to include that plot.
> http://snapon.lab.bufferbloat.net/~d/beagle_bql/beaglebonewithbql.png
> You can disregard the decline in download bandwidth (as we are also
> sending 5x as many acks and measurement data, which is not counted
> in that part of the plot)
>> This is partially due to the improvement in ack clocking you get from
>> reduced RTTs, partially due to improved cache behavior (shorter
>> queues), and partially continual improvements elsewhere in the tcp
>> portions of the stack.
>> With more recent kernels...
>> I now get full throughput from the beagles in both directions with the
>> 3.16 kernel,
>> the stil out of tree bql patch, and either fq or fq_codel. I haven't
>> got around to plotting all those results (they are from kathie's new
>> lab), but they are here:
>> http://snapon.lab.bufferbloat.net/~d/pollere/
> The latency spikes are generally due to not having BQL, probably:
> http://snapon.lab.bufferbloat.net/~d/pollere/beagle/beagle-3.8-nobql-fq-fq_codel.png
> This is using the new fq scheduler on both sides, with BQL enabled.
> http://snapon.lab.bufferbloat.net/~d/pollere/beagle/beagle_3.16-fq-fq-no-offloads.png
> The switch most likely is prioritizing EF marked packets. (as is
> sch_fq). Most of the buffering is in the switch, not the host, now.
> (the prior results I showed had no switch in the way)
>> There is a small buffered tail dropping switch in the way, on these
>> later data sets. There was some puzzling behavior on the e1000e that I
>> need to poke into in a more controlled setting.
>> As for other tunables on hosts, TCP small queues might be amiable to
>> some tuning, but that too may well evolve further in kernelspace.
> So I have now drowned you in data on one architecture and setup. The
> most thoroughly publicly analyzed devices and drivers are the ar71xx,
> e1000e, and beaglebone at this point.
> The use of fq_codel in a qos system (artificially rate limited using
> htb, hfsc, or tbf) is pretty well proven to be a huge win at this
> point.
> At line rates fq and fq_codel still help quite a bit without a BQL
> enabled driver, BQL is needed for best results. I don't know to what
> extent the BQL enabled drivers already cover the marketplace, it was
> generally my assumption that the e1000e counted for a lot...
> http://www.bufferbloat.net/projects/bloat/wiki/BQL_enabled_drivers
> And thus with all the positive results so far, more wider distribution
> of the new stuff
> on more devices outside the sample set of those on the bloat,
> cerowrt-devel and codel lists (about 500 people all told),
> and all ya gotta do is turn it on.

Thanks for the very detailed info Dave.

What I'm taking from this is that at the moment we should not be doing
anything more than what we already do (the sysctl changes that Michal
pushed), and that any improvements should be made in the kernel
drivers (BQL support in particular).

What we could do in the future, is in case you have kernel features
that should be enabled by default as they benefit the general user,
but where you cannot change the kernel defaults due to backwards
compat, we can (as we did with fq) add these to the recommended sysctl
files shipped with systemd. This is of course just a gentle nudge for
people to use features, distros/admins can still override them.

> This gets me to the stopping point that we hit a whlle back, which was
> reliably determining if a good clocksource was present in the system.
> Somehow. clock_getres, perhaps?

I appear to have missed the context on this. In what situation do we
(userspace) need to care about the existence of a high quality



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