netdev - Re: [PATCH net-next v3 0/7] net: sched: pie: align PIE implementation with RFC 8033

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Date:   Mon, 25 Feb 2019 17:02:12 -0800
From:   Dave Taht <dave.taht@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Leslie Monis <lesliemonis@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Mohit P . Tahiliani" <tahiliani@...k.edu.in>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH net-next v3 0/7] net: sched: pie: align PIE implementation
 with RFC 8033

On Mon, Feb 25, 2019 at 4:38 PM Stephen Hemminger
<stephen@...workplumber.org> wrote:
>
> On Tue, 26 Feb 2019 00:39:54 +0530
> Leslie Monis <lesliemonis@...il.com> wrote:
>
> > The current implementation of the PIE queuing discipline is according to the
> > IETF draft [http://tools.ietf.org/html/draft-pan-aqm-pie-00] and the paper
> > [PIE: A Lightweight Control Scheme to Address the Bufferbloat Problem].
> > However, a lot of necessary modifications and enhancements have been proposed
> > in RFC 8033, which have not yet been incorporated in the source code of Linux.
> > This patch series helps in achieving the same.
> >
> > Performance tests carried out using Flent [https://flent.org/]
> >
> > Changes from v2 to v3:
> >   - Used div_u64() instead of direct division after explicit type casting as
> >     recommended by David
> >
> > Changes from v1 to v2:
> >   - Excluded the patch setting PIE dynamically active/inactive as the test
> >     results were unsatisfactory
> >   - Fixed a scaling issue when adding more auto-tuning cases which caused
> >     local variables to underflow
> >   - Changed the long if/else chain to a loop as suggested by Stephen
> >   - Changed the position of the accu_prob variable in the pie_vars
> >     structure as recommended by Stephen
> >
> > Mohit P. Tahiliani (7):
> >   net: sched: pie: change value of QUEUE_THRESHOLD
> >   net: sched: pie: change default value of pie_params->target
> >   net: sched: pie: change default value of pie_params->tupdate
> >   net: sched: pie: change initial value of pie_vars->burst_time
> >   net: sched: pie: add more cases to auto-tune alpha and beta
> >   net: sched: pie: add derandomization mechanism
> >   net: sched: pie: update references
> >
> >  include/uapi/linux/pkt_sched.h |   2 +-
> >  net/sched/sch_pie.c            | 107 ++++++++++++++++++++-------------
> >  2 files changed, 66 insertions(+), 43 deletions(-)
>
> Are you concerned at all that changes to default values might change
> expected behavior of existing users?

There's existing users?

Most of these changes are really subtle and came out as we went along.
I'd have to *really search* to find my notes from that period,
and I have not evaluated the sum of these new changes, but overall
they were a slight improvement from the defaults as we shipped it way
back when.

I'm not being snarky - are there existing users? Every last person I
know that ever evaluated pie switched to fq_codel. Cisco ended up
going with another bufferbloat-fighting solution in one new device and
that team dissolved.

docsis-pie, well, that's a standard. and that's a bit different from
this pie, but this pie is closer to that pie. And it works ok.

I certainly hope leslie's group has done similar testing to what we
did then ? If there's results I can dig up my old results and see what
compares. I imagine they plan a paper. our old flent test code and
data is all published, but am not willing to spend another minute of
my life on a single queued aqm design unless someone writes me a very
big check for it, which I'd then spend on fixing P4 to run fq-codel.

fq-pie (which is waiting in the wings behind this set of patches) is
ok, as at least, the BSD version was competetive with fq_codel in most
respects. I told 'em they needed to look hard at the rate estimator.

The ECN handling problem mentioned is there on all known versions of
pie, but it's that it reverts to drop too soon for ecn to be
as much benefit as it could be.

and then l4s, which is the subject of 3 upcoming talks at netdevconf,
well... not gonna talk about it. I'm willing to give them their day in
court.

sorry to come across as grumpy. should linux be rfc compliant even if
the rfc has issues? damned if I know.

-- 

Dave Täht
CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-205-9740