[Bloat] when does the CoDel part of fq_codel help in the real world?

[Luca Muscariello]

I think that this is a very good comment to the discussion at the defense
about the comparison between
SFQ with longest queue drop and FQ_Codel.

A congestion controlled protocol such as TCP or others, including QUIC,
LEDBAT and so on
need at least the BDP in the transmission queue to get full link
efficiency, i.e. the queue never empties out.
This gives rule of thumbs to size buffers which is also very practical and
thanks to flow isolation becomes very accurate.

Which is:

1) find a way to keep the number of backlogged flows at a reasonable value.
This largely depends on the minimum fair rate an application may need in
the long term.
We discussed a little bit of available mechanisms to achieve that in the
literature.

2) fix the largest RTT you want to serve at full utilization and size the
buffer using BDP * N_backlogged.
Or the other way round: check how much memory you can use
in the router/line card/device and for a fixed N, compute the largest RTT
you can serve at full utilization.

3) there is still some memory to dimension for sparse flows in addition to
that, but this is not based on BDP.
It is just enough to compute the total utilization of sparse flows and use
the same simple model Toke has used
to compute the (de)prioritization probability.

This procedure would allow to size FQ_codel but also SFQ.
It would be interesting to compare the two under this buffer sizing.
It would also be interesting to compare another mechanism that we have
mentioned during the defense
which is AFD + a sparse flow queue. Which is, BTW, already available in
Cisco nexus switches for data centres.

I think that the the codel part would still provide the ECN feature, that
all the others cannot have.
However the others, the last one especially can be implemented in silicon
with reasonable cost.





On Mon 26 Nov 2018 at 22:30, Jonathan Morton <chromatix99 at gmail.com> wrote:

> > On 26 Nov, 2018, at 9:08 pm, Pete Heist <pete at heistp.net> wrote:
> >
> > So I just thought to continue the discussion- when does the CoDel part
> of fq_codel actually help in the real world?
>
> Fundamentally, without Codel the only limits on the congestion window
> would be when the sender or receiver hit configured or calculated rwnd and
> cwnd limits (the rwnd is visible on the wire and usually chosen to be large
> enough to be a non-factor), or when the queue overflows.  Large windows
> require buffer memory in both sender and receiver, increasing costs on the
> sender in particular (who typically has many flows to manage per machine).
>
> Queue overflow tends to result in burst loss and head-of-line blocking in
> the receiver, which is visible to the user as a pause and subsequent jump
> in the progress of their download, accompanied by a major fluctuation in
> the estimated time to completion.  The lost packets also consume capacity
> upstream of the bottleneck which does not contribute to application
> throughput.  These effects are independent of whether overflow dropping
> occurs at the head or tail of the bottleneck queue, though recovery occurs
> more quickly (and fewer packets might be lost) if dropping occurs from the
> head of the queue.
>
> From a pure throughput-efficiency standpoint, Codel allows using ECN for
> congestion signalling instead of packet loss, potentially eliminating
> packet loss and associated lead-of-line blocking entirely.  Even without
> ECN, the actual cwnd is kept near the minimum necessary to satisfy the BDP
> of the path, reducing memory requirements and significantly shortening the
> recovery time of each loss cycle, to the point where the end-user may not
> notice that delivery is not perfectly smooth, and implementing accurate
> completion time estimators is considerably simplified.
>
> An important use-case is where two sequential bottlenecks exist on the
> path, the upstream one being only slightly higher capacity but lacking any
> queue management at all.  This is presently common in cases where home CPE
> implements inbound shaping on a generic ISP last-mile link.  In that case,
> without Codel running on the second bottleneck, traffic would collect in
> the first bottleneck's queue as well, greatly reducing the beneficial
> effects of FQ implemented on the second bottleneck.  In this topology, the
> overall effect is inter-flow as well as intra-flow.
>
> The combination of Codel with FQ is done in such a way that a separate
> instance of Codel is implemented for each flow.  This means that congestion
> signals are only sent to flows that require them, and non-saturating flows
> are unmolested.  This makes the combination synergistic, where each
> component offers an improvement to the behaviour of the other.
>
>  - Jonathan Morton
>
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