Re: [Bloat] benefits of ack filtering

[Neal Cardwell <ncardwell@google.com>]

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On Thu, Nov 30, 2017 at 5:24 AM, Eric Dumazet <eric.dumazet@gmail.com>
wrote:

> I agree that TCP itself should generate ACK smarter, on receivers that
> are lacking GRO. (TCP sends at most one ACK per GRO packets, that is
> why we did not feel an urgent need for better ACK generation)
>
> It is actually difficult task, because it might need an additional
> timer, and we were reluctant adding extra complexity for that.
>

How about just using the existing delayed ACK timer, and just making the
delayed ACK logic a bit smarter? We could try using the existing logic and
timers, but using something adaptive instead of the magic "2" MSS received
to force an ACK.


> An additional point where huge gains are possible is to add TSO
> autodefer while in recovery. Lacking TSO auto defer explains why TCP
> flows enter a degenerated behavior, re-sending 1-MSS packets in
> response to SACK flood.
>

Yes, agreed. I suspect there is some simple heuristic that could be
implemented to allow TSO deferral for most packets sent in recovery. For
example, allowing TSO deferral once the number of packet bursts (TSO skbs)
sent in recovery is greater than some threshold. Perhaps TSO deferral would
be fine in Recovery if we have sent, say, 10 skbs, because at that point if
the ACK stream from the original flight dries up due to massive/tail loss,
we have probably sent enough data in the new flight in Recovery to ensure
some kind of ACKs come back to keep the ACK clock going.

neal


>
>
> On Thu, 2017-11-30 at 09:48 +0200, Jonathan Morton wrote:
> > I do see your arguments.  Let it be known that I didn't initiate the
> > ack-filter in Cake, though it does seem to work quite well.
> > With respect to BBR, I don't think it depends strongly on the return
> > rate of acks in themselves, but rather on the rate of sequence number
> > advance that they indicate.  For this purpose, having the receiver
> > emit sparser but still regularly spaced acks would be better than
> > having some middlebox delete some less-predictable subset of them.
> > So I think BBR could be a good testbed for AckCC implementation,
> > especially as it is inherently paced and thus doesn't suffer from
> > burstiness as a conventional ack-clocked TCP might.
> > The real trouble with AckCC is that it requires implementation on the
> > client as well as the server.  That's most likely why Google hasn't
> > tried it yet; there are no receivers in the wild that would give them
> > valid data on its effectiveness.  Adding support in Linux would help
> > here, but aside from Android devices, Linux is only a relatively
> > small proportion of Google's client traffic - and Android devices are
> > slow to pick up new kernel features if they can't immediately turn it
> > into a consumer-friendly bullet point.
> > Meanwhile we have highly asymmetric last-mile links (10:1 is typical,
> > 50:1 is occasionally seen), where a large fraction of upload
> > bandwidth is occupied by acks in order to fully utilise the download
> > bandwidth in TCP.  Any concurrent upload flows have to compete with
> > that dense ack flow, which in various schemes is unfair to either the
> > upload or the download throughput.
> > That is a problem as soon as you have multiple users on the same
> > link, eg. a family household at the weekend.  Thinning out those acks
> > in response to uplink congestion is a solution.  Maybe not the best
> > possible solution, but a deployable one that works.
> > - Jonathan Morton
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