netdev - Re: [PATCH net-next v12 3/7] sch_cake: Add optional ACK filter

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Date:   Thu, 17 May 2018 04:04:54 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Toke Høiland-Jørgensen <toke@...e.dk>,
        netdev@...r.kernel.org
Cc:     cake@...ts.bufferbloat.net
Subject: Re: [PATCH net-next v12 3/7] sch_cake: Add optional ACK filter

On 05/16/2018 01:29 PM, Toke Høiland-Jørgensen wrote:
> The ACK filter is an optional feature of CAKE which is designed to improve
> performance on links with very asymmetrical rate limits. On such links
> (which are unfortunately quite prevalent, especially for DSL and cable
> subscribers), the downstream throughput can be limited by the number of
> ACKs capable of being transmitted in the *upstream* direction.
> 

...

> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@...e.dk>
> ---
>  net/sched/sch_cake.c |  260 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 258 insertions(+), 2 deletions(-)
> 
>

I have decided to implement ACK compression in TCP stack itself.

First step is to take care of SACK, which are the main source of the bloat,
since we send one SACK for every incoming out-of-order packet.

These SACK are not only causing pain on the network, they also cause
the sender to send one MSS at a time (TSO auto defer is not engaged in this case),
thus starting to fill its RTX queue with pathological skbs (1-MSS each), increasing
processing time.

I see that your ACK filter does not take care of this common case :)

Doing the filtering in TCP has the immense advantage of knowing the RTT and thus be able
to use heuristics causing less damage.