netdev - Re: [PATCH net-next] fq_codel: avoid under-utilization with ce

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Date:   Sun, 31 Oct 2021 13:31:27 -0700
From:   Dave Taht <dave.taht@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Neal Cardwell <ncardwell@...gle.com>,
        Asad Sajjad Ahmed <asadsa@....uio.no>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Ingemar Johansson S <ingemar.s.johansson@...csson.com>,
        Tom Henderson <tomh@...h.org>,
        Bob Briscoe <research@...briscoe.net>,
        Olga Albisser <olga@...isser.org>
Subject: Re: [PATCH net-next] fq_codel: avoid under-utilization with
 ce_threshold at low link rates

On Fri, Oct 29, 2021 at 7:53 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Fri, Oct 29, 2021 at 6:54 AM Neal Cardwell <ncardwell@...gle.com> wrote:
> >
> > On Thu, Oct 28, 2021 at 3:15 PM Asad Sajjad Ahmed <asadsa@....uio.no> wrote:
> > >
> > > Commit "fq_codel: generalise ce_threshold marking for subset of traffic"
> > > [1] enables ce_threshold to be used in the Internet, not just in data
> > > centres.
> > >
> > > Because ce_threshold is in time units, it can cause poor utilization at
> > > low link rates when it represents <1 packet.
> > > E.g., if link rate <12Mb/s ce_threshold=1ms is <1500B packet.
> > >
> > > So, suppress ECN marking unless the backlog is also > 1 MTU.
> > >
> > > A similar patch to [1] was tested on an earlier kernel, and a similar
> > > one-packet check prevented poor utilization at low link rates [2].
> > >
> > > [1] commit dfcb63ce1de6 ("fq_codel: generalise ce_threshold marking for subset of traffic")
> > >
> > > [2] See right hand column of plots at the end of:
> > > https://bobbriscoe.net/projects/latency/dctth_journal_draft20190726.pdf
> > >
> > > Signed-off-by: Asad Sajjad Ahmed <asadsa@....uio.no>
> > > Signed-off-by: Olga Albisser <olga@...isser.org>
> > > ---
> > >  include/net/codel_impl.h | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/net/codel_impl.h b/include/net/codel_impl.h
> > > index 137d40d8cbeb..4e3e8473e776 100644
> > > --- a/include/net/codel_impl.h
> > > +++ b/include/net/codel_impl.h
> > > @@ -248,7 +248,8 @@ static struct sk_buff *codel_dequeue(void *ctx,
> > >                                                     vars->rec_inv_sqrt);
> > >         }
> > >  end:
> > > -       if (skb && codel_time_after(vars->ldelay, params->ce_threshold)) {
> > > +       if (skb && codel_time_after(vars->ldelay, params->ce_threshold) &&
> > > +           *backlog > params->mtu) {
>
> I think this idea would apply to codel quite well.  (This helper is
> common to codel and fq_codel)
>
> But with fq_codel my thoughts are:
>
> *backlog is the backlog of the qdisc, not the backlog for the flow,
> and it includes the packet currently being removed from the queue.
>
> Setting ce_threshold to 1ms while the link rate is 12Mbs sounds
> misconfiguration to me.
>
> Even if this flow has to transmit one tiny packet every minute, it
> will get CE mark
> just because at least one packet from an elephant flow is currently
> being sent to the wire.
>
> BQL won't prevent that at least one packet is being processed while
> the tiny packet
> is coming into fq_codel qdisc.
>
> vars->ldelay = now - skb_time_func(skb);
>
> For tight ce_threshold, vars->ldelay would need to be replaced by
>
> now - (time of first codel_dequeue() after this skb has been queued).
> This seems a bit hard to implement cheaply.
>
>
>
>
> > >                 bool set_ce = true;
> > >
> > >                 if (params->ce_threshold_mask) {
> > > --
> >
> > Sounds like a good idea, and looks good to me.
> >
> > Acked-by: Neal Cardwell <ncardwell@...gle.com>
> >
> > Eric, what do you think?
> >
> > neal

My 2c:

I would prefer this entire patch series be reverted and put back into
the l4s and BBRv2 trees where it can be thoroughly evaluated, and
appropriate parameterizations found for bare metal DCs, virtualized
environments, hosts and routers at a variety of bandwidths.

I'm not huge on arbitrarily cluttering up the fq_codel hot path, or
the uapi, before that happens.

Also, if anyone has any suggestions as to how to apply this correctly
to the wifi users in the kernel, I'm all ears.

I think a safer path forward in linux mainline would be to remove ECT0
from consideration as RFC3168 ECN in the INET_macros (which all the
existing qdiscs will then pick up) and have the unconfigured linux
hosts and vm substrates across the internet revert to drop in that
case. Prague is defined to revert to reno in that case. Throughput, if
not reductions in local queuing, will be good.

A good test of the ECT0 concept across the wire might be to then have
the in-kernel dctcp instance start using that instead of ECT1 and see
what breaks.

I would appreciate a set of before/after benchmarks on a future submission.

It's otherwise not my call, I do wish more folk would read this (
https://github.com/heistp/l4s-tests#key-findings) before committing
seemingly untested code.

-- 
I tried to build a better future, a few times:
https://wayforward.archive.org/?site=https%3A%2F%2Fwww.icei.org

Dave Täht CEO, TekLibre, LLC